英文文法問題 呢個一定高手先識

What I really want to do(It is a noun clause which functions as a noun and answers the question"What?" It is used as a subject in the sentence you stated.)

What I really want to do( It is used as a subeject) is to ask (verb) her what she has been doing for the past fifteen years.

e.g. What the fisherman caught earlier (It is used as a subject) was merely on old shoes.
e.g. Your job is to really understand these problems.

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You can use
[what I really want to do is ask]
or
[what I really want to do is to ask]

but you cant use
[what I really want to do is asking].

The reason [to do is ask] is OK, is that the [to] needed for [ask] is the same [to] from [to do]. Its shorter and people understand you if you just use [to] once.

what i really want to do is TO ASK her what she has been doing for the past fifteen years. -
It means that you have not asked her as you have been thinking to ask her.

This is more appropriate

Is ask is wrong in written English but people understand what you mean.

Somehow, asking or ask are both acceptable in any conversation because
people understand what you want to express. Also, you can treat it
like imperatives.