Question

if unchecked they will expand again (rewrite as negative)

Answer 1

If it is not checked they will expand again.

Answer 2

They will expand again if they are unchecked.

Normally, the negative sentence in this case is the sentence that states that something is false.

One of the example of an auxiliary verb is the helping verb 'be.'

In English, we create negative sentences by adding the word 'not' after the auxiliary, or helping, verb.