I am setting an exam at my school and this sentence is causing a lot of problems. 'All we did was make it worse.' Many colleagues have said that it is grammatically wrong as 'was' is a linking verb. I know the sentence is correct but can't explain why. Can anyone help me???

All we did was make it worse.

This is a good question, and I had to think about it for quite a while. I am prepared to be corrected. I think the sentence is similar in structure to:

He is a hero
i.e. noun (subject) - linking verb - noun (predicate)


I'll explain why.

Look at this sentence.

Cows sleeping in a field is something you don't see often.

Here, the subject is "Cows sleeping in a field". That's why it's singular. Note the use of is not are.

I make this point to demonstrate that All we did is your subject.

"make it worse" is the infinitive form of make. You could equally have written to make, but I judge the to to be unnecessary. As you know, infinitive verbs can used as nouns. (e.g. His ambition was to dance.") Therefore, "make it worse" is playing the role of a noun.

So, you have

Noun (All we did)
Verb (was)
Noun (make it worse)

And, there's nothing wrong with that. Even if this theory gets slaughtered by others that follow, I would say this:

You're right. This sentence is correct. If we can't make the grammar fit, then that's the bit we need to fix not the "answer", which the whole English-speaking world would understand and use.

The tricky bit here is that there are several verbs, only one of which is the main verb.
"All we did" -- this noun phrase serves as the subject.
"Was" is your main verb. It's a linking verb, but there's nothing that says an infinitive verb can't follow a linking verb.
"make it worse" is an infinitive verb phrase (infinitives often have "to" in front of the verb, but it gets dropped).
This construction is called a "pseudo-cleft sentence" and is perfectly acceptable in English.

Brilliant. Thanks, GrammarMonkeys.