1. I am studying accounting, which I find difficult.
2. The house which was built in 1950 is still nice.
I'm confused when it comes to adjective clause or relative pronous, why is the first sentence with comma and the other without.
If it is extra information, surround it with commas. If it is information which is required in order to identify the noun (accounting, house), then no commas.
PS: I would use commas for #2 under normal circumstances.