What they were doing was bathing in a tub.
What I can do for you is call for a taxi.
I don't know why the baby is crying. All I did was smile at her.
In there examples the verb after be will take the from that from the verb in the what-clause is normally followed by:
What I want is to sleep.
What he hates is getting up early.
This is how you " cleave" a verb which is in the past simple and present perfect:
What I did in the end was go home.
What I have done is write a letter to the editor.