Study materials
In both sentences the stress falls somewhere other than where the usual order would put it.
What we need is more time.
She did call you.
In the first, the known part is wrapped in a what clause and the new part comes last, where the stress is strongest. In the second, do before the verb stresses that it really is so. This article explains sentences with what and all, the reversed order, the choice between what and it, and emphatic do.
Sentences with what
Here the order is reversed. The known part sits in the what clause, and the new part comes after be, at the end, where it sounds strongest.
What we need is a clear deadline.
What surprised everyone was the price.
This pattern can also highlight the action itself. Then the what clause has do, and be is followed by the base form of the verb, the form with no endings. To before it is possible but less common.
What she did was rewrite the whole section.
All I want is a straight answer.
What happened was that the server crashed.
If the what clause has an -ing form, so does the part after be: What I'm doing is checking the figures. All instead of what adds and nothing more. What happened was that highlights a whole event.
The reversed order
A sentence with what can be turned round. Then the highlighted part comes first and the what clause last.
What we requested was a full refund.
The usual order, the new part last.
A full refund is what we requested.
The reversed order, the highlighted part first.
The reversed order is natural when the highlighted word has already been mentioned. It picks it up and confirms it.
Which to choose
The choice depends on where the main point should stand.
| Pattern | Where the main point is | When it fits |
|---|---|---|
| It was X who… | at the start | correcting someone, contrasting |
| What… is X | at the end | leading to a conclusion, introducing something new |
A sentence with it does not highlight the action itself: It was rewrite the section that she did is impossible. A sentence with what does not highlight a time or place, because what means a thing. A time or place takes it: It was in March that we changed it.
Do before the verb
When someone doubts or denies something, do, does or did goes into a positive sentence. The meaning stays the same, but it sounds insistent.
I did tell you.
She does know the answer.
Do come in.
A warm invitation.
After do, does or did the main verb takes the base form, because do already shows the tense and the -s: did tell, does know. In an imperative do makes an invitation warmer.
Common mistakes
The first mistake is a past form after did.
I did told you. → I did tell you.
Did already shows the tense.
The second mistake is an extra it after the what clause.
What I need it is a holiday. → What I need is a holiday.
The what clause is already the subject.
The third mistake is a past form after what she did was.
What she did was rewrote the report. → What she did was rewrite the report.
The base form follows was here.
The first and third mistakes show in the verb. After did and after What ... did was it takes its base form, because the past is already shown. The second shows where the two parts meet. The what clause is already the subject, so a second it is extra.