Day Questions

If one knows without a doubt that he himself is a sinner, but people do not know this and consider him pious and ask him to lead prayers, is it permissible for him to do so and will their prayers be accepted?

It is not permissible for him to present himself to lead the congregational prayer if he knows that he does not fulfil the requirements of the Imam of the prayer, in such a way he would be considered as cheating them.If they asked him to lead the prayer, it is permissible to do so, although it is better not to in this particular case as well. If he was praying alone and someone decided to pray behind him in congregation, then his prayer is in order and he has not committed a sin. If he was not qualified to lead the prayer, he should not apply the laws of the congregational prayer on him, for example, referring to the people praying behind him when he has doubts.

Which takbeers are obligatory and which are recommended in prayers?

The obligatory takbeer is only the takbeerat'ul-ihtam. The rest of the takbeers are recommended and not obligatory.