As salamualaikum warahmatullahi wabarakatahu.
Let us learn about the conditions which are required to fast in Ramadan. These conditions are:
1) To be a Muslim. First and foremost condition, to do it only for the sake of Allah 
The fast of a disbeliever is not valid. If he becomes Muslim he does not have to make up fasts from before.
The evidence for that is the verse in which Allaah says (interpretation of the meaning): “And nothing prevents their contributions from being accepted from them except that they disbelieved in Allaah and in His Messenger (Muhammad) and that they came not to As-Salaah (the prayer) except in a lazy state, and that they offer not contributions but unwillingly” [al-Tawbah 9:54]
If the contribution is not acceptable even though it benefits others, because of their kufr, then other acts of worship may be even more unacceptable.
He does not have to make up fasts if he becomes Muslim because Allaah says (interpretation of the meaning): “Say to those who have disbelieved, if they cease (from disbelief), their past will be forgiven” [al-Anfaal 8:38]
2) To have reached puberty. Puberty is that period during which adolescents achieve sexual maturity or become capable of reproduction.
3) To be sane.
Ali bin Abi Talib (ra) and Aisha (ra) both reported that Prophet Muhammad 
said,
” The pen is raised from (the book) of three (individuals): the sleeper until he awakes, the child until he reaches puberty, and the insane until he regains his sanity.”(Sahih Muslim, Eng. translation, Vol.2, Pp 552-553, hadith nos. 2531-2532)
Thus, the fasting person amongst the muslims should be accountable (mukallaf). The one who is mukallaf is one who is has reached the age of puberty and is of sound mind, because a minor or one who is insane is not accountable.
The one who is of sound mind is the opposite of one is insane, which is one who has lost his mind, whether he is insane or feeble-minded. Everyone who has lost his mind, in whatever sense, is not accountable and he is not obliged to do any of the obligatory duties of Islam, be it prayer, fasting or feeding the poor; he does not have to do anything at all.
4) To be physically able. The one who is unable to fast does not have to fast. But inability to fast comes under two categories: Temporary inability and permanent inability. These will be discussed later in sha Allah under exemptions from fasting.
5) To be settled (as in, not travelling) The case of a traveller will also be discussed soon in sha Allah.
6) There should be no impediments. This applies specifically to women. Women who are menstruating or bleeding following childbirth should not fast.Their fasting is not valid under these conditions, but they have to make up the missed fasts. If a woman who is menstruating or bleeding following childbirth becomes pure during the daytime in Ramadan and she was not fasting, she does not have to refrain from eating and drinking, and she may eat and drink, because refraining will not benefit her in any way, as she is obliged to make up that day.
Discussion on exemption from fasting:
1. Journey: For a traveller,
- If fasting and not fasting are the same, in the sense that fasting does not affect him, then in this case fasting is better.(Fasting whilst traveling means that one fulfils one’s duty more quickly, because making it up later means delaying it, but fasting in Ramadaan means doing it sooner.It is usually easier for the one who has this duty, because fasting and breaking the fast with the people is easier than starting to fast all over again.It makes the most of a blessed time, namely Ramadaan, for Ramadaan is better than other times, because it is the time when fasting is obligatory. Based on this evidence the view of al-Shafaa’i, which is that fasting is better in the case of one for whom fasting and not fasting are the same, is most likely to be correct.)
- If not fasting is easier for him, then in this case we say that not fasting (when traveling) is better. If something will give him hardship, then in his case fasting becomes makrooh, because doing something that causes hardship when there is a concession indicates that one is spurning a concession granted by Allaah.
- If it will cause unbearable difficulty, then in this case it becomes haraam for him to fast. The evidence for that is the report narrated by Muslim from Jaabir ibn ‘Abd-Allaah (may Allaah be pleased with them), that the Messenger of Allaah (peace and blessings of Allaah be upon him) went out to Makkah in the year of the Conquest in Ramadaan, and fasted until he reached Kuraa’ al-Ghameem. The people were fasting, but he called for a cup of water and lifted it up so that the people could see it, then he drank it. After that, he was told that some of the people had continued to fast. He said, “Those are the disobedient, those are the disobedient.” According to another report, he was told, “The people are finding it hard to fast, and they are waiting to see what you will do.” So he called for a cup of water after ‘Asr. (1114) So he described those who fasted even though it was very difficult as being disobedient.
The one who is traveling for sinful purposes is not allowed to break his fast or avail himself of any of the other concessions granted to travellers.
2)Pregnant or Lactating women:
The pregnant woman must be in one of two situations:
The first is that she is active and strong, and faces no difficulty, and her foetus is not affected. Such a woman is obliged to fast, because she has no excuse not to fast.
In the second case, the pregnant woman is not able to fast, either because the pregnancy is difficult for her, or she is physically weak, or some other reason. In this case she may not fast, especially if fasting will harm her foetus; in that case it is obligatory for her to refrain from fasting.
If she does not fast then she is like others who do not fast because of some excuse; she must make up the fasts when that excuse is no longer applicable in her case. When she has given birth, she must make up the fast after she becomes pure following the end of nifaas (postpartum bleeding). But sometimes the excuse of pregnancy may cease, but it is immediately followed by another excuse, which is the excuse of breastfeeding. The breastfeeding mother may need to eat and drink, especially during the long days of summer when it is very hot. She may need to break the fast in order to be able to nourish her baby with her milk. In this case she may break the fast, and when that excuse is no longer applicable to you, you should make up the fasts that you missed.
It is known that this concession – granted to pregnant and breastfeeding women – is limited to cases where they fear for themselves or for their children.
If their excuse is that they fear for the child, then they must make up the missed fasts when they are able to, and according to some scholars they also have to feed one poor person for each day missed, giving wheat, rice, dates or any other staple food if they delay making up the fasts without a legitimate reason.
3) Sickness:
Most of the scholars – including the four imams – are of the view that the sick person is not allowed to break his fast in Ramadaan unless the sickness is severe.
What is meant by severe sickness is:
1- The sickness will be made worse by fasting
2- Recovery will be delayed by fasting
3- Fasting will cause intense hardship, even if it does not make the sickness worse or delay recovery
4- The scholars also included those who fear that they may become sick because of fasting.
Shaykh Ibn ‘Uthaymeen said: “The sick person who is not affected by fasting, such as one who has a slight cold or headache, or a slight toothache and the like, is not permitted to break his fast.”
Allaah has permitted the sick to break the fast in Ramadaan and make it up later on, as He says (interpretation of the meaning):
“So whoever of you sights (the crescent on the first night of) the month (of Ramadan i.e. is present at his home), he must observe Sawm (fasts) that month, and whoever is ill or on a journey, the same number [of days which one did not observe Sawm (fasts) must be made up] from other days”
[al-Baqarah 2:185]
This applies if the sickness is one from which it is hoped that the person will recover.
If the sickness is one from which there is no hope of recovery – in the doctors’ opinion – then he may break the fast and feed one poor person for each day.
If a sick person does not fast, and his sickness is one from which there is no hope of recovery, and he feeds one poor person for each day, then Allaah heals him, he does not have to make up the fasts, because he did what was required of him, and he discharged his duty thereby.
4) The very old:
They have to feed one poor person for each day, giving half a saa’ of the local staple food, whether it is dates, rice or something else. This is equivalent to approximately one and a half kilograms. This was the fatwa of a number of the companions of the Prophet (peace and blessings of Allaah be upon him), including Ibn ‘Abbaas (may Allaah be pleased with him).
The evidence for that is the report narrated by Abu Dawood (2318) from Ibn ‘Abbaas concerning the verse (interpretation of the meaning):
“And as for those who can fast with difficulty, (e.g. an old man), they have (a choice either to fast or) to feed a Miskeen (poor person) (for every day)”
[al-Baqarah 2:184]
[Ibn ‘Abbaas] said: “This was a concession granted to old men and women who are able to fast but with difficulty, they have the option of not fasting and feeding one poor person for each day instead.”
If they are poor and cannot feed others then they do not have to do anything. It is permissible to give this kafaarah (expiation) to one or to more than one person at the beginning of the month, or in the middle or at the end.
5) Jihad:
If those who are fighting the disbelievers have travelled the distance which allows them to shorten their prayers, then it is permissible for them not to fast, but they have to make it up after Ramadaan.
If they are not travelling because the disbelievers have attacked them in their own land, then whoever among them is able to fast whilst fighting is obliged to fast, and whoever is not able to combine fasting with the duty of jihad which has become an individual obligation (fard ‘ayn) for him is permitted not to fast, but he must make it up later on, fasting the number of days that he did not fast in Ramadaan.
If a person is about to participate in Jihad and fears that the fasting would weaken him, he may defer the fast. It is permissible for the mujaahideen to break their fast in Ramadaan, so that they will have strength for jihad, even if they are in their own country, for fasting weakens their ability to fight and deal with the enemy.
Muslim (1120) narrated that Abu Sa’eed al-Khudri (may Allaah be pleased with him) said: We traveled with the Messenger of Allaah (peace and blessings of Allaah be upon him) to Makkah – meaning at the Conquest of Makkah – and we were fasting. We stopped to camp and the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “You are approaching your enemy and breaking the fast will make you stronger.” This was a concession, and some of us continued to fast and some of us broke our fast. Then we stopped to camp again, and he said, “You are going to meet your enemy in the morning, and breaking the fast will make you stronger, so break your fast.” So we had no choice but to break our fast.
Abu Dawood (2365) narrated that one of the companions of the Prophet (peace and blessings of Allaah be upon him) said: I saw the Messenger of Allaah (S) telling the people on a journey in the year of the Conquest (of Makkah) to break their fast. He said, “Gain strength to face your enemy.”
Al-Haafiz said in al-Talkhees al-Habeer: this was classed as saheeh by al-Haakim and Ibn ‘Abd al-Barr
These two hadeeths indicate that the command to break the fast was not because of travelling, rather it was in order to gain strength for jihad.
It says in al-Muntaqa Sharh Muwatta’ al-Imam Maalik:
The words “Gain strength to face your enemy” indicate that this was the reason breaking the fast. If the reason was travelling then he would not have explained it as being in order to gain strength to face the enemy, rather he would have said it was because of travelling.
And Allah knows best.
As salamualaikum warahmatullahi wabarakatahu.
LIFE 