Adding a Rak'ah by Mistake
Question :

I performed four rak’ah of prayers instead of three for the Maghrib prayer, by mistake. Is there a way to rectify my prayer, or must I offer the prayer again?

Answer :

Your prayer is invalid in this case and you are obligated to perform it again correctly.

Forgetting the Rak'ah
Question :

Most of the time while performing prayers of four rak'ahs, I forget which rak'ah I was performing, whether it was the third or the fourth. In such a situation what can I do?

Answer :

You should assume it is the fourth rak'ah. And after finishing the prayer with Tasleem, you should perform a prayer of one rak'ah - as the prayer of precaution - in which only Surah al-Fatiha is recited, and then you bow down, prostrate twice, and end with Tashahhud and Tasleem. Alternatively, you can perform this prayer as explained in two rak'ahs while in the sitting position.

Doubt in Prayer
Question :

When one has a doubt during the prayer, can he break the prayer and start again?

Answer :

It is permissible to stop the prayer and start again when doubts occur. However, the prayer can be corrected in some cases of doubts, without the need to start again.

Doubts in the Validity of the Prayer
Question :

What is the position of the validity of a prayer performed by somebody who did all the actions for the prayer, bowing and prostrations quickly and pronouncing the words incorrectly? What must he do now?

Answer :

He can consider the prayer as valid if he doubts the validity now and if there is a probability that it might have being valid.

Prostrations of Forgetfulness
Question :

Should we do the two prostrations of forgetfulness on mistakes in recommended prayers? Is it obligatory or recommended in this situation?

Answer :

It is not obligatory, but it is better to do it.