My brother attends notre dame university. the main clause is?

There is only one clause in this sentence. The entire sentence, in this case, functions as a standalone clause. There isn't a second clause to append to the sentence or to separate the sentences. A clause such as "He first thought he was going to attend Vincennes University, but..." might be considered sufficient to warrant the sentence having two clauses.

Hey perhaps you should add the passage? then we'll be able to you! : )