You're right.
correct: The crabs which we caught yesterday are still alive.
The crabs are still alive- main clause/principal clause
Which we caught yesterday- relative clause/subordinate clause/dependent clause
The crabs which we caught yesterday are still alive.
The relative clause 'which we caught yesterday'pronoun 'which' refers to 'the crabs'. ItThe relative clause 'which we caught yesterday' gives us more information about 'the crabs'. We put it just after 'the crabs'.
We can leave out 'which'.
The crabs we caught yesterday are still alive.