Hi, excuse me for re-posting my question, I haven't got the answer, well the final one.

The question is, I can't tell when I need to put to's before verbs in the sentences like below:
All you have to do is ask a question.
All I want to do is stay home tonight.
The goal of this lecture is "to" explain how routing between...
My challenge is "to" explain why Israel is preferred.

So, will you explain when to's are required?

Thanks in advance.
The first two are pseudo-clefts with do. The to is therefore optional.
The last two are not pseudo-clefts. They are ordinary equatives, so the to is required.

See (to) be.

Offhand I can't think of any uses of "is + bare infinitive" (e.g. "is ask", "is stay", "is explain") except following the verb "do", as in your examples.

I'm not certain though. It's very difficult to instantly bring to mind every possible circumstance in which this combination of words might be used.
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I see. Still you make sense. I also can't come up with the example besides these do-cases. Well, thanks for your help, always, kinda solved.
 CalifJim's reply was promoted to an answer.
Thanks for the reply and the link, you blow the fog away!
Students: We have free audio pronunciation exercises.
jimmiefujitayou blow the fog away!
Sounds good. We don't want fog now, do we? Emotion: wink

Nope, of course, we don't.