What is the penalty for dressing one's father in a dirty (impure) garment, with which he prayed unknowingly and unintentionally, and how can one atone for that?

The answer includes several points:

1. You should have informed your father about the impurity of his garment. Otherwise, you have committed a sin from which you must repent, unless you feared significant harm from informing him.

2. Your prayer behind your father while you knew about the impurity of his garment: If you were aware of the ruling, your prayer is invalid and must be repeated. If you were unaware, it is valid according to some scholars.

3. Your father's and mother's prayers are valid if they did not know about the impurity of the garment, or if they became aware of it after the prayer. This is the view of the majority of scholars.

4. The prayer of a sitting imam is valid for one who is able to stand, and he should pray standing behind him. This is the view of many scholars.