Btukfyl

What is the passive voice of “they did not do the work”?

The work was not done by them.

In such a sentence one is " Dummy Do", the meaningless one the support of which we seek in Simple Present and Simple past where there is no auxiliary or modal but the sentence is to be made negative or interrogative.

Ignore this first DO, the meaningless one, we call do support.

• They ( did) NOT DO the work.

We know 'do support' embody the tense and person of the finite verb. Thus deduced, the sentence is in simple past.

Let's make it passive.

• The work was not done by them.

We know BE Verb in a sentence is conspicuous by its presence in the sense that it precedes the subject in questions and turns negative all by itself without support of any helping verb, irrespective of the fact that BE VERY can be main verb or auxiliary.

It is definitely:

The work wasn't done(by them).

We form the passive using the auxiliary "to be" + "past pariticiple". (by them) is optional since we usually focus on the action when we use the passive.

The difference between "they did not do the work" and "the work was not done by them" is the latter does not convey that the work was not done, and implies someone else may have. The question that follows is "well who did it?"

I prefer a more specific choice of words to more clearly create the inference that the work was not done at all:

the work was overlooked (by them).

The work did not done by them.

The anwser is:
The work was not made by them