Prayer is invalidated by:
1.Among the invalidators of the prayer is for one to speak words people usually converse with one another. This excludes making dhikr in remembering Allah, which does not invalidate the prayer.
If a person intentionally utters two letters, even if these letters do not have a meaning or if he utters just one letter that has a meaning, while remembering that he is in prayer, this invalidates his prayer.
There are some words in Arabic that have a meaning and are composed of just one letter.
For example, the letter ‘f’ pronounced with a kasrah, (i.e. ‘Fi’) means "keep your word or promise". If a person utters this letter intentionally, during his prayer, while remembering he is praying his prayer is invalidated.
This judgment applies to the one who is aware that speaking in prayer (other than the words spoken for prayer) is a forbidden matter. It does not invalidate the person's prayer if he speaks a little while praying because he forgets that he is praying.
2.Among the invalidators of prayer is for one to make many moves during the prayer that are other than the actions of the prayer. Some scholars said moving three consecutive moves or moving three body parts at once invalidates the prayer. Other scholars said the moves that invalidate the prayer are those that last for a period of time which is equal to the duration of one cycle of the prayer. This saying has a stronger evidence.
3.If one performs one excessive move while in prayer, ones prayer is invalidated. For example, if one jumped.
4. If one intentionally performs an extra integral of action one's prayer is invalidated. For example, if one performed the bowing, straightened for the standing, and, while remembering, performed an additional bow, one's prayer would be invalidated.
5. Moving, even slightly, while praying with the purpose of playing invalidates the prayer, because the intention of playing conflicts with the intention of praying.
6. If a person eats or drinks while remembering he is praying, his prayer is invalidated. His prayer is invalidated, even if he eats or drinks a very little amount, like, if he intentionally swallows the food stuck between his teeth.
7. Having the intention to interrupt the prayer invalidates it. For example if one says to oneself, "I now interrupt my prayer", then one's prayer is invalidated immediately upon the intention.
8. If a person makes the interruption of his prayer contingent upon the happening of a certain matter, then his prayer is invalidated immediately upon his intention. For example, if a person decides that he will interrupt his prayer to answer the door should the doorbell ring, then his prayer is invalidated immediately--whether or not the doorbell rings.
9. As well, if one hesitated or wavered in that matter, like if he was praying and hesitated thinking to himself, "If a thief comes in now while I am praying, should I interrupt my prayer or not?", then his prayer would be invalidated.
10. If a person doubts whether or not they established their intention to perform the prayer during the opening takbir (saying Allaahu akbar), and this doubt lasts for the duration of an integral, the prayer is invalidated.
For example, if a person doubts about his intention while reciting the verses in the Faatiha, then goes to bowing, while still in doubt, once he reaches the bow with the doubt still in his heart, his prayer will be invalidated.
Similarly, if a person doubted about his intention for a long time, even if he did not complete an integral, then this would invalidate his prayer. For instance, if a person was reciting a verse after the faatiha, before performing the bow, and while reading these verses, he doubted about the intention if this doubt lasted for a long time during that recitation, this would invalidate his prayer.
If one doubted about the intention but cleared oneself of that doubt quickly, that is, before the termination of the integral, ones prayer would not be invalidated.
We ask Allah to make us listen and comprehend. We ask Allah to help us live our lives in a way that is accepted and rewardable allahuma Ameen
Source: from a book authored by Shk Abdullah al-Harari