Subjunctive after adjectives

I assume that the first sentence is there for a reason, and that you wish to understand why it allows both forms while the second allows only one.

The reason is that insist has two meanings, or two shades of meaning, if you will.

If you insist that someone goes to Athens, you are saying quite firmly that you know that the person makes regular trips to and from Athens.  The person goes (regularly, as a habit) to Athens, and you know it, and you state it firmly, possibly in the face of resistance from others who do not believe it.

If you insist that someone go to Athens, you are in deliberative mode.  A decision is being made whether the person should go to Athens (make one trip there).  You envision the person traveling to Athens.  That envisioned act is what you firmly want to happen, and you argue in favor of it.

On the other hand, it is advisable has no meaning that allows a similar interpretation.  You can't say, for example,

He spends the holidays with you (regularly, habitually), and that's what is advisable / what I advise.

(It's too late to advise anything once you already know the facts.)

Therefore, the second sentence means that the spending of holidays with you is an envisioned act -- and that this envisioned act should become a reality in the future, i.e., that it is advisable for it to become a reality.  Envisioned acts are often expressed in the subjunctive.  An alternative is the FOR ... TO ... construction:

It is advisable for him to spend the holidays with you.

Does this help?
CJ

Sorry for butting in.

You know what! We’re only taught to learn it by rote this list of nouns and verbs “demand, proposal, suggestion; suggest, advise, order, command, insist, require, request……” And are never told the semantic meaning in common of all these words. “Envisioned act” has never been mentioned by them. What a pity!

This meaning of insist expresses serious possibility, does it not?

No.  Not to me.  It expresses conviction and certainty.

She insists that she  is right.  (She knows that she is right, and says so with great conviction.)

There is a clear and definite difference between

1. Mother insists that Johnny take his medicine.

and

2. Mother insists that Johnny takes his medicine.

In 1, we are likely at home with Johnny and his mother.  Johnny may be putting up a fuss, not wanting to take his medicine, but Mother says he has to do it.  She insists on it.  She is not going to take "No" as an answer.  She insists that he take his medicine.

In 2, we are likely in the doctor's office.  The doctor sees that Johnny is not getting better.  He questions Johnny's mother about whether Johnny has been taking his medicine.  Mother says with great conviction, "My son takes his medicine three times a day as prescribed".  She insists that he takes it.

CJ

"It expresses conviction and certainty." This is the key to my problem.
THanks Jim.