How “Is + verb” possible?

Question

We can use a full infinitive or a gerund as a complement of the 'be'-verb. For example,

The first thing I do in the morning is to check my mobile phone.

The first thing I do in the morning is checking my mobile phone.

But I've seen the bare infinitive used in such construction with no real difference:

The first thing I do in the morning is check my mobile phone.

Thus, bare infinitive is used when the finite element of the first verb is a form of "do" : Using bare infinitive after 'does'.

Now, my question is whether it's a case of elision, such as "The first thing I do in the morning is [that I] check my mobile phone." or is there any other grammatical explanation?

Answer 1

The first thing I do in the morning is check my mobile phone

This is perfectly grammatical and perhaps the most common. When the subject has a form of "do", the infinitive is used. You can put "to" there, but it is optional.

The first thing I do in the morning is checking my mobile phone

The sentence above is not grammatical. It'll only be grammatical if the verb "do" is progressive as in:

The first thing I was doing was checking my mobile phone.