1) What are the conditions that make a Salat completely void i.e. there’s no obligation of Sajda e Sehw and one should perform that salat again?

2)Is qunoot in second rakat wajib or mustahab? If someone forgets reciting it is he/she obligatory to perform the salat again or is he/she to perform sajda e sehw or it doesn’t make that salat void?

by

1) https://www.sistani.org/urdu/book/61/3638/
2) Qunoot is mustahab. Without it, Namaz is ok.