about "to infinitive"

[learnerr]

Hi, I'm a student majoring in English grammar and got some questions

I know that in some cases "to-infinitive" is used as a noun after the intransitive verb. (as a kind of an object for intransitive verb)

Like "I agree to leave", "He failed to make his case" (quoted from Wikipedia)

The question is,

In the case of "He tends to be unhappy" , is 'to-infinitive' also used as a noun-like infinitive? just like the ones used in the examples above?

Or is 'tend to do' just one of special phrases like 'have to do, ought to do, get to do"?

Hope you'd answer me!

[TheParser]

Hi, I'm a student majoring in English grammar and got some questions

I know that in some cases "to-infinitive" is used as a noun after the intransitive verb. (as a kind of an object for intransitive verb)

Like "I agree to leave", "He failed to make his case" (quoted from Wikipedia)

The question is,

In the case of "He tends to be unhappy" , is 'to-infinitive' also used as a noun-like infinitive? just like the ones used in the examples above?

Or is 'tend to do' just one of special phrases like 'have to do, ought to do, get to do"?

Hope you'd answer me!

***** NOT A TEACHER *****


(1) I have checked my books, and I believe the following to be

accurate.

(2) First, "He tends to be unhappy" is NOT the same as so-called

"complementary infinitives": I have to go; I am going to study; I used

to work there. As Descriptive English Grammar points out, those

three sentences could be expressed as: I must go; I shall study; I once

worked there. It is my opinion that "He tends to be unhappy" does not

fit that pattern.

(3) Yes, you are 100% correct. We could say that in "I agree to leave,"

"to leave" is the direct object of the verb "agree." Here's another

example from Descriptive English Grammar: He helps (to) support his

parents.

(4) Re: "He tends to be unhappy," both Descriptive English Grammar

and A Comprehensive Grammar of the English Language point out that

"tend to" is similar to "seem to."

In other words, the infinitive "to be unhappy" has the function of an

adjective. It is a subjective complement. Obviously, you cannot say:

He tends. You need a complement (the word "complement" comes from

the word "complete") to complete the meaning of the sentence. Thus, we

can probably conclude that the infinitive phrase "to be unhappy" is a

subjective complement -- NOT an object.

[learnerr]

Thank you so much, your reply truly helped me out.
Makes me feel free now.
Thanks for your help!