Q: In the third and forth rakat of salah behind an imam, do I recite surah fathiha, you said to do always do so. (student)

A: False. (if you are ahnaf)

True (if you are shawafee)

Allah Certainly Knows Best.

Note: If you don’t know, then merely be silent whilst the imam is reading anything loudly.

The above applies to fard salah only. For non fard salah, you would recite surah fatiha in all rakats coupled with a surah.