Question

he spoke only necessary words(turn into complex)​

Answer 1

Only necessary words are spoken by him.

Answer 2

He spoke the words whenever necessary.

(or)

He spoke the words that are only necessary.

EXPLANATION:

There are three types of sentences. Simple, compound and complex sentences. Simple sentences have only an independent clause. Example: Jack reads the newspaper. Compound sentences have two independent clauses joined together by conjunctions like and, but, for, etc., Example: I like an apple but my sister likes banana.

Complex sentences have one independent clause and one dependent clause joined by subordinate conjunction like although, because, even though etc., Example: Even though she came late, she finished her homework. In the given sentence we can use ‘whenever’ because he spoke only the necessary words.