Grammar for Everyone - Barbara Dykes 2007

Adverbial clause of reason
Introduction
Part II The parts of speech

This clause tells why something is done, or happens.

Finite verbs:  screamed, dropped

Main clause:  Bella screamed

Conjunction:  because

Subordinate clause: a spider dropped on her arm

Adverbial clause of reason, modifying the verb ’screamed’ in the main clause.

Complex sentence