Hello again, kynguyen
You said:
"In fact I think most of the verbs can be used with or without "to", right?
For ex: I need to have a vacation. Or I need a vacation."
You used the verb "need" as an exampe of the verbs that can be used with or without the particle "to".
But the particle "to", when it is part of an infinitive, will precede the verb, not follow it.
In "I need to take a vacation" the infinitive is "to take", whereas "need" is a conjugated verb in the simple present tense.
In your second and third examples, "I need a vacation" and "I need the money for my vacation", there is no infinitive at all. The verb "need" is again in the simple present, and the constructions that follow the verb, in both sentences, are noun phrases (the main word in those constructions is a noun). That is why no "to" is needed. Actually, the use of "to" either before or after "need" in those two sentences would be incorrect.
Your fourth example, "I need to taking a vacation" is also incorrect, as was pointed out in the previous post. The reason, again, is that "to" has to appear
before the verb when it is part of an infinitive. That is the reason why this type of infinitive is called "to-infinitive": the name shows the order in which both the particle "to" and the verb must appear, and that order can't be altered.
In "I need to taking a vacation", "to" does not "belong" to "need". It is part of the following verb, "take", which should be a "to-infinitive" (not an -ing form); so "taking" is incorrect. The infinitives, in English, are the "base" form of the verb, the verb as you would find it in a dictionary entry. The ending "-ing" is added to a verb after the particle "to"
only when "to" is a preposition.
Have a look at these sentences, in which "to" precedes "take" and, together with it, forms a "to-infinitive":
"I have
to take a vacation."
"I need
to take a vacation."
"I want
to take a vacation."
"I would like
to take a vacation."
In all the above examples, "to" does not have a meaning and is only use as part of an infinitive verb. It is different from the "to" used in "I'm looking forward to seeing you soon", where it is a preposition and has a meaning of its own.
Miriam