Answer:
I guess what you did last night really wasn't that interesting.
The subordinate clause: what you did last night
Type: Noun clause
Step-by-step explanation:
A subordinate clause is a group of words containing a subject and a verb that does not express a complete thought, therefore, they can not stand alone a full sentence. There are various types of subordinate clauses, one of them are noun clauses.
A noun clause act as a noun in a sentence, that is to say, as a subject, a direct or indirect object, a predicate nominative, or an object of a preposition, and they tend to begin with the words what, how, that, whatever, when, where, whether, which, whichever, who, whoever, whom, whomever, or why.
In the sentence provided, then, the subordinate clause is what you did last night because it has a subject and a verb and it does not express a complete thought on its own. Furthermore, it is also a noun clause because it begins with the word what and is the direct object of the sentence, that is to say, the noun that receives the action guess in the sentence.