Question

Throughout history, people have turned great ideas into useful inventions.

Which word is the direct object?

Answer 1

The answer is ideas. Because, what is your action? Your action is turned. What was turned? Ideas of course. 

Answer 2

In the sentence "Throughout history, people have turned great ideas into useful inventions" the word ideas is the object direct.

The direct object of a verb is the thing being acted upon. A direct object comes after a transitive verb. Direct objects can be nouns, pronouns, phrases, or clauses. Sometimes direct objects are single words.

Direct objects can also follow verbals—infinitives, gerunds, and participles.