Answer:
Adjective clause: who was English
Word(s) modified: William Shakespeare
Step-by-step explanation:
A clause is a group of word that have at least a subject and a verb, and that can be either dependent, which means that can not stand on its own and express a complete thought, or independent, which means that it does not need to depend on another clause in order to express a complete thought.
Dependent clauses can function as an adjective, adverb or a noun. In the sentence, "who was English" is a clause because it has a subject and a verb, it is dependent because it does not express a complete thought, and it is an adjective clause as it modifies, describes or gives further information of the noun: William Shakespeare.