Which form of a verb should I normally use after "what you have done is"? Should it be present participle (option A), past participle (option B) or a base form (option C) :
A. I wanted you to clean the whole bathroom, but what you have done is simply washing the sink.
B. I wanted you to clean the whole bathroom, but what you have done is simply washed the sink.
C. I wanted you to clean the whole bathroom, but what you have done is simply wash the sink.
Any of them can be used, though they come about in different ways.
C. is the canonic Pseudo-Cleft (aka Wh-Cleft) construction, with an infinitive VP wash the sink as the complement of the Active pro-verb do required by pseudo-clefting. Note that one can optionally insert (or delete, if one goes to a different syntactic church) the infinitive complementizer to in front of wash.
B. is a variant pseudo-cleft, a reduction of what you have done is (you have) washed the sink, with the past participle washed being required after the perfective have.
A. is perhaps the least likely. In it, the gerund phrase washing the sink (you can tell it's really a gerund and not just a noun because it has a direct object) is the complement of what you have done. Note that they can be swapped, salva veritate:
English has thousands of such constructions, and each has its own set of affordances, prohibitions, requirements, and idioms. In many cases they are effectively synonymous; in others they can contrast. This is why we have such a variety; most of them are still in beta.
If you rephrase the statement this way:
Q: I wanted you to clean the whole bathroom, but what have you done?
A: You have simply washed the sink (the past participle, a participle that typically expresses completed action per Merriam-Webster).
So:
I wanted you to clean the whole bathroom, but what you have done is simply washed the sink.
