I didn't know what time it was, so I was very late to class.

Explanation

- I didn't know what time it was (Independent clause)- so I was very late to class (Dependent clause, introduced by the subordinating conjunction "so")The dependent clause "so I was very late to class" cannot stand alone as a complete sentence, and the independent clause "I didn't know what time it was" provides the reason for the dependent clause, making the sentence compound-complex