A mumin must strive to pray namaz with absolute focus and concentration because he is in the presence of Allah. However, it is inevitable that he will sometimes forget or make mistakes; mistakes in namaz are of two kinds:

1.      He may forget one of the arkaan (ruk‘u, sujud etc) and be certain that he has forgotten it. This is known as sehw.

      2.    He may believe he has forgotten something but is not sure of it. This is known as shak.

Situations in which one should repeat namaz (namaz valvi aave)

1.      When one forgets to say takbirat ul ehram.

2.      When one forgets to recite Surat tul Fateha (al Hamd).

3.      When one is in doubt whether he has prayed Surat ul Fateha.

4.      When one forgets to perform ruk‘u.

5.      When one gets up for a 5th raka‘at without praying mohto tashahud after 4 raka‘at, or if one gets up for 4th raka‘at in Maghrib, or in the 3rd raka‘at in fajar and performs ruk‘u.

6.      If one deliberately doesn’t recite the surat after al Hamd.

7.      If one performs more than one ruk‘u in one raka‘at.