the verb "to have"

@clueless posted:

I understand that "have to" is a modal verb so that means that it's followed by a main verb.

Hi, clueless—"Have to" is not a modal exactly. Modals don't change form or have tense. But "have to" inflects for tense and number: he has to; they have to; he had to; they had to.

"Have to" does have modal meaning and it is commonly referred to as a "phrasal modal"—sometimes also as a "semi-modal" or "quasi-modal"—along with "be about to," "be going to," "be supposed to," "used to," etc.

@clueless posted:

How is the modal verb "have to" distinguishable from the main verb "have" followed by an infinitive? Thanks.

A) I have to shine my shoes.
B) I have a new car.
C) I have to do my homework.

Syntactically, there is nothing wrong with treating "have to" as the verb "have" complemented by an infinitival clause; however, in spoken English, "have to" is normally pronounced with the "to" contracted: hafta, hasta, hadda.

Your examples (A) and (C) both feature the phrasal modal. It is only (B) that has "have" with its full-verb meaning. Analyzing the relationship between "have to" and main-verb "have" is not a walk in the park. It's a historical question.

Briefly, the phrasal modal originated in a related construction which we still have, in which the the direct object is followed by an infinitival. Over time, the possibility of the phrasal-modal construction emerged as an alternative.

• I have a horse to sell.
• I have to sell a horse.

There is a difference in meaning between those two constructions. "I have a horse to sell" means that I own a horse and I could sell it; in contrast, "I have to sell a horse" means that I am obligated to sell a horse.

In an article I read once on the evolution of "I have to sell a horse" from "I have a horse to sell," it was pointed out that a key evolutionary stage was when the object of "have" could refer to something which does not yet exist!

• I have a paper to write.
• I have to write a paper.