What is the simple subject of how can we put a difficult puzzle piece together with some pieces missing?

The simple subject of the sentence "How can we put a difficult puzzle piece together with some pieces missing?" is we.

Here's why:

* Subject: The subject of a sentence is the person, place, or thing that performs the action.

* Simple Subject: The simple subject is the main noun or pronoun in the subject.

* "We" is the pronoun that is performing the action of "putting a difficult puzzle piece together."

The rest of the sentence is the predicate, which tells us what the subject is doing.