Question

Identify the main clause in the sentence below. Then decide if the subordinate clause is used as noun, adjective, or adverb. The clams, which he ate, were still fresh.

Answer 1

In the following sentence

The clams, which he ate, were still fresh.

The main clause is:

The clams..., were still fresh.


The subordinate clause is then "which he ate".

This clause gives more information about The clams, which means it has the function of an adjective: it's an adjective clause. 

Answer 2

The main clause in the sentence The clams, which he ate, were still fresh is The clams were still fresh.

The subordinate clause, that is which he ate, is used as an adjective, since it comes after the noun and motifies it. This type of clauses are usually introduced by words like that, when, where, which, etc. and separated by commas whenever it is a non-defining clause.