If a person does not pray at all (zero), he is a kafir and not a Muslim.

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Prayer is valid with just the recitation of al Fatiha, although one is missing out on a sunnah by not reciting another surah. The prayer is valid.

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You should make isthikhara each time you have taken a decision and place your trust in Allah. Please watch this video about when to pray Istikhara: https://www.assimalhakeem.net/when-to-pray-istikhara/ and this one: https://www.youtube.com/watch?v=msaehIgYBJ0

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