I need help understanding simple subjects and simple predicates. The sentence is: What is keeping them?

Question

I'm confused about which is the simple subject and simple predicate.
Thank you

Answer 1

http://www.chompchomp.com/terms/subject.htm
The simple subject is the noun or pronoun that is the subject without including any adjectives or phrases that modify it.

http://www.chompchomp.com/terms/verb.htm
The simple predicate (verb) is the main verb of the sentence, including any auxiliary verbs, but excluding any objects, complements, or modifiers.

In your sentence "What" is the simple subject.

In your sentence "is keeping" is the simple predicate.

Answer 2

The first thing to do is find the verb. In this sentence the verb (simple predicate) is is keeping.

Then ask who or what is doing this action? Which is the only word that answers that question? That is the subject.

I'll be glad to check your answer.