Infinitive without to: The first thing I do is open my eyes

I have not been able to find an explanation for this use of an infinitive without to:

The first thing I do in the morning is go to the bathroom.

The first thing I do in the morning is open my eyes.

The first thing I do in the morning is turn off the alarm clock.

Infinitives without to are used in the following cases:

-After modal auxiliary verbs (We can managed it)

-After do (I do admit I was wrong)

-After certain verbs like let, make, see... (They made me wait)

-After rather, better and had better (I would rather go alone)

-After and, or, except, but, than, as and like (It is as easy to smile as frown)

-After why (Why pay more at other shops?)

[ http://www.perfectyourenglish.com/grammar/infinitives-without-to.htm ]

I have found no mention of using an infinite without to after the verb to be. Are the examples above a special case? Are they very colloquial? Or are they incorrect?

Solution 1:

In this case actually infinitive with and without "to" is possible (and I was surprised that "to" is possible here, as it sounds quite uncommon to me).

From bbc.co.uk:

There are one or two other structures where to-infinitive and the bare infinitive are both possible. Expressions with do or did, such as what I've done or all I did can follow either pattern.

I hate shopping so what I've done is (to) order a new computer over the Internet.

All I did was (to) suggest that she should lend him no more money. I didn't insist on it.

The examples you gave are not incorrect in any way, and such structures are commonly used.

Solution 2:

The infinitive without to is called the bare infinitive in English. The places where one can use the bare infinitive in English are a multitude, far too many to list exhaustively, but you've listed many of the major ones above. None of your examples are incorrect. Rather, they're all perfectly grammatical and natural statements, acceptable in both colloquial and formal communication.

Solution 3:

Would you say this?

This is the first thing I do in the morning. To go to the bathroom.

This is the first thing I do in the morning: to go to the bathroom.

I wouldn't.

If anything, it's the fact that the sentence

The first thing I do in the morning is to go to the bathroom.

(see Vilmar's answer) is acceptable that might need the explanation.

If you want the best rule of English grammar:

Don't accept every grammatical rule you find in a book of grammar without a lot of careful consideration.

And a few close contenders:

Every rule has exceptions. If one seems not to have any, it's because nobody has thought of one yet.

You'll probably find another rule in another book of grammar that contradicts the one you're looking at.

Nobody has yet written the perfect grammar.