"insist on" vs "insist"

Question

The definition of the insist on phrasal verb told us that it means to keep doing something, even if it annoys other people, or people think it is not good for you.

In contrast the definition of the just insist verb is to say firmly or demand forcefully, especially when others disagree with or oppose what you say.

I'd like to clarify the difference between them. As far as I understood insist on assumes that we're already doing something, but just insist told us that we're just saing firmly about our intention to do something. So, in the sentences with insist:

I don't know why you insist on talking about it.

The listener is already talking about that "it".

I don't know why you insist to talk about it.

The listener's just saying firmly to talk about it, but not talking yet.

Is it correct?

Answer 1

Your second sentence sounds rather awkward.

I don't know why you insist to talk about it.

A native speaker would internally understand you to mean

I don't know why you insist on talking about it.

When you say insist on, this should be following by a verb describing what it is the particular individual continues to do. Though there doesn't have to be anything about annoying anyone, it could just describe a situation which is futile in the mind of the speaker.

I don't know why you insist on watering the flowers. (Perhaps, they're dead already).

But insist without on is usually going to be followed by a particular statement asserted by the one doing the insisting.

John insisted that the flowers be watered every day regardless of whether they're alive or not.

In other words, the suggestion here is that John probably said, "I want the flowers to be watered every day."

Answer 2

The construction insist + infinitive is at the very least "non-standard", and many native speakers would probably say it's "ungrammatical". The standard format is insist on + present participle.

So it should be, for example "I insist on knowing the truth", not "I insist to know the truth". OP's idea that there might be some semantic difference is simply mistaken - it's just a matter of standard vs non-standard syntax.

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Note that in constructions such as "She insisted [that] he stayed for lunch", most careful speakers would use the subjunctive form "She insisted [that] he stay for lunch". Like the infinitive, which has the same form, the subjunctive doesn't "conjugate" (for different tenses, or first/second/third person subject, etc.).