What is the subject?

[Dearleader]

In this example what is the subject of this sentence?

Suppose he stole a gun and committed a crime with it.

What is this type of construction called and what is the subject of suppose?

[Raymott]

In this example what is the subject of this sentence?

Suppose he stole a gun and committed a crime with it.

What is this type of construction called and what is the subject of suppose?

This is an imperative sentence:
"[You] suppose he stole a gun ..."
The implied subject is "you". This is the case generally for imperative sentences.

Some imperative sentences have specific subjects:
"Charles, eat your breakfast!" Here the subject is "Charles".