Using bare infinitive after 'does'

All he does is watching TV.

I said this sentence and my native friend corrected me and said 'you need to use bare infinitive here'

All he does is watch TV.

Why is this right while the other is false? If I change the placement of the complement and subject, I will have this sentence:

Watch TV is all he does.

Shouldn't it be 'watching TV (or to watch) is all he does'?

I searched for it online and I couldn't get anything. Is there a rule that I'm missing? When do they use bare infinitive after 'to be' verbs?

Thanks

Answer

Your friend's correction was accurate. A native speaker would say

All he does is watch TV.

Your proposed reversal of complement and subject is also correct: it would indeed be expressed as

Watching TV is all he does.

"To watch TV is all he does" is grammatically correct, but you would almost never hear it in normal speech.

Unfortunately, there is no universal rule governing this usage. However, "does" is almost always followed by the bare infinitive in everyday usages like this one, even when another verb (like "to be" in your example) comes between them.

A thread at the Wordreference.com forum discusses this usage.

The omission of "to" before a complemental infinitive ("to watch" in your example") is expected when the finite element of the first verb is a form of "do" or a modal verb with "do" as dependent:

All he does is watch TV.
All he did was watch TV.
All he could do was watch TV.

The omission of "to" is optional when the finite element of the first verb is a non-modal verb with "to do" as its complement:

All he wants to do is (to) watch TV.

Like many things in English, this initially confusing construction will become more clear as you use the language in everyday conversation and reading.