What are the conditions of a valid fast?

Two kinds of conditions

Scholars separate the conditions that make fasting obligatory (who has to fast) from the conditions that make a fast valid (what makes a day of fasting count). It helps to keep them apart.

Who has to fast

• Muslim: fasting is an act of worship for those who have accepted Islam.
• Adult: having reached puberty. Children are encouraged but not obliged. See At what age do Muslim children have to fast?
• Sane: someone who has lost their reason is not held responsible.
• Able: healthy enough to fast. The sick may postpone.
• Resident: travellers may postpone and make up the days later.

What makes a fast valid

• Intention (niyyah): the Prophet ﷺ said actions are judged by intentions (Sahih al-Bukhari 1). For an obligatory fast, most scholars say the intention must be made at night before dawn, and it is simply a decision in the heart. See Do I have to make an intention to fast every night?
• Avoiding what breaks the fast: no deliberate eating, drinking or sexual intercourse from true dawn to sunset.
• Being free of menstruation and post-natal bleeding: a woman's fast is not valid during these days, and she makes them up later. See Can women fast on their period?
• Fasting in the right time: fasting on the two Eid days is not allowed.

What doesn't affect validity

Many small things worry new Muslims for no reason. Forgetfully eating, swallowing your own saliva, a shower, a wet dream, or waking up in a state of major impurity (as long as you make ghusl for prayer) do not cancel your fast. The Hanafi, Maliki, Shafi'i and Hanbali schools differ on some details, such as whether one intention at the start of Ramadan covers the whole month, but they agree on the core above. See Does swallowing my spit break my fast?

Also remember that the fast is meant to change your behaviour. Lying, backbiting and anger don't technically cancel the fast, but they can empty it of reward.