this is my first time using this website to ask a question. I hope you can help me!

I work as an English assistant teacher, and during a lesson, I pinged this sentence

“What we must do is helping him”

As incorrect. One of my co-teachers got it from a workbook. However, I couldn’t explain why this was incorrect, and I don’t have a lot of grammar experience. All I had was a ping that it sounded a bit incorrect to my ear. I offered either “What we must do is help him”, or “Helping him is what we must do” as alternatives. Due to the way the students are learning this grammar point, it is important that the “What” is at the start of the sentence.

Can anyone help with explaining how to fix “What we must do is helping him”? Or confirm if it’s correct or not? Thank you!

  • Thanks very much! It is indeed a cleft sentence. The reasons you gave really helped clear it up.
    – Jardo
    Jun 23, 2020 at 4:21

2 Answers 2


The sentence is grammatical, but not in the sense that you think.

A description of current events


"What must we do?"
"You must hold the door for him when he is carrying large packages and doesn't have a hand free to do so himself."


"What me must do (hold the door) is helping him."

Or to paraphrase with additional wording:

"The thing that we must do, and that we're doing, is actually helping him."
"What we were told to do in order to help him is actually helping him."

Or to simply replace the word:

"What we must do is benefiting him."

So, in this sense of the phrase:

✔ What we must do is helping him.

An instruction of future obligation

On the other hand, if what you're trying to express is not a description of how things are working out, but a directive about what must be done, then you need to rephrase it:

✔ What we must do is help him.

  • Thank you so much! The examples really cleared it up. You were totally right, it is grammatical in that sense, but that’s definitely not the sense I was intending.
    – Jardo
    Jun 23, 2020 at 4:17

It is not correct. The correct versions would be: What we must do is help him. What we must do is to help him. Although, personally, the first one sounds better to my ear. I believe that there is a typo in the workbook. Books are not infallible and do contain mistakes.

  • be+infinitive. According to M. Swan in Practical English Usage, p 87, you use this structure (be+infinitve) for: plans and arrangements, fate, pre-conditions and orders. Jun 23, 2020 at 4:47

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