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?)
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?