We use the to-infinitive:

• to express purpose (to answer "Why...?"):

He bought some flowers to give to his wife.
He locked the door to keep everyone out.

We sometimes say in order to or in order not to:

We set off early in order to avoid the traffic.
They spoke quietly in order not to wake the children

… or we can say so as to or so as not to:

We set off early so as to avoid the traffic.
They spoke quietly so as not to wake the children.

• after certain verbs (see verbs followed by infinitive), particularly verbs of thinking and feeling:

choose, decide, expect, forget, hate, hope, intend, learn, like,
love, mean, plan, prefer, remember, want, would like, would love

… and verbs of saying:

agree, promise, refuse

They decided to start a business together.
Remember to turn the lights out.

Some verbs are followed by a direct object and the infinitive(see verbs followed by infinitive):

advise, ask, encourage, invite, order, persuade, remind, tell, warn,
expect, intend, would prefer, want, would like

She reminded me to turn the lights out.
He encouraged his friends to vote for him.

• after certain adjectives.

Sometimes the to-infinitive gives a reason for the adjective:

  • disappointed
  • glad
  • sad
  • happy
  • anxious
  • pleased
  • surprised
  • proud
  • unhappy

We were happy to come to the end of our journey
= We were happy because we had come to the end of our journey
John was surprised to see me
= He was surprised because he saw me

Other adjectives with the to-infinitive are:

  • able
  • unable
  • due
  • eager
  • keen
  • likely
  • unlikely
  • ready
  • prepared
  • unwilling
  • willing

Unfortunately I was unable to work for over a week.
I am really tired. I’m ready to go to bed.

We often use the to-infinitive with these adjectives after it to give opinions:

  • difficult
  • easy
  • possible
  • impossible
  • hard
  • right
  • wrong
  • kind
  • nice
  • clever
  • silly
  • foolish

It’s easy to play the piano, but it’s very difficult to play well.
He spoke so quickly it was impossible to understand him.

We use the preposition for to show who these adjectives refer to:

  • difficult
  • easy
  • possible
  • impossible
  • hard

It was difficult for us to hear what she was saying.
It is easy for you to criticise other people.

We use the preposition of with other adjectives:

It’s kind of you to help.
It would be silly of him to spend all his money.

• As a postmodifier (see noun phrases) after abstract nouns like:

  • ability
  • desire
  • need
  • wish
  • attempt
  • failure
  • opportunity
  • chance
  • intention

I have no desire to be rich.
They gave him an opportunity to escape.
She was annoyed by her failure to answer the question correctly.

• We often use a to-infinitive as a postmodifier after an indefinite pronoun (See indefinite pronouns):

When I am travelling I always take something to read.
I was all alone. I had no one to talk to.
There is hardly anything to do in most of these small towns.




Hello alyuuv!
You're right that to is a prepostion sometimes, but there is also a verb form called the infinitive. This is often used with to, but it is not the same as the preposition. It is a special grammar form, as in 'I was unable to work.' In English grammar, we can't say 'I was unable work', we use 'to +verb' instead.

We often use this infinitive form with adjectives to show what we think of something - give our opinions. For example:
English grammar is sometimes difficult to understand.
is an opinion - what I think about English grammar.
I hope that helps!
Jeremy Bee
The LearnEnglish Team

And also, most of my teachers(not a native) says that in these sentences, to infinitives and participle phrase is used to indicate a 'reason'.
He must be crazy to leave now.(or leaving)
(My teachers said 'to' acts like 'because')
But, above, there is a just one explanation, that is"give a opinion". And also, there is a no explanation in the book "advanced grammar in use".

Hello again alyuuv!
The grammar description does say that infinitives can be used to give reasons - look again:

• after certain adjectives.
Sometimes the to-infinitive gives a reason for the adjective.
Jeremy Bee
The LearnEnglish Team

1.Researchers at Iowa State University are testing how well catnip oil works to do the same thing.
In this sentence, does 'to do the same thing' works as an adverb? or a noun? If it works as an adverb, it contains the meaning of purpose, or intent?
2.He grew up to be a good pianist.
In this sentence, to-infinitive is used as an adverb. My tutor says 'To' roles as 'and'.
3.To see it, you will not believe it.
In this sentence, to infinitive is used like 'even if', accoding to my KOREAN grammar book. But, my pal says that that's a wrong sentence. Is that wrong?

I would agree with your pal - the sentence 'To see it, you will not believe it' sounds very unnatural to me.
Best wishes,
The LearnEnglish Team

Hi Sir,
I have confusion in using( to Preposition) with verb.
Example : somebody uses the describe verb as " to describe" somebody uses as " to describing " what is the deference between two. and in which instance we can use the above sentence. please explain. 
 ( Same as " to accept " and  " to accepting"
 Thanking you

Hello mohamed!
I'm sorry, but without seeing the whole sentences you took your examples from, it's very hard to explain why one sentence uses 'to describe' and one uses 'to describing'. I think you are asking about the difference between to + verb and to (preposition) + verb-ing. You can have a look at my reply to alyuuv below, and see if that helps you understand. If you can give me a bit more information, I might be able to help you more.
Jeremy Bee
The LearnEnglish Team

"There was nobody to talk to."
"I hated to be told what to do."
"The job was too difficult to be finished alone."
These things happened in the past, so why is the infinitive in the present tense ("to talk", "to be told", "to be finished")?
I understand that it is correct, but how can I explain it to someone? :)

Hello weezer!
The infinitive isn't in the present tense - it's just the infinitive form, and stands outside the tense system. You can just as well say "There will be nobody to talk to", and you will still use the infinitive.
Hope that helps!
Jeremy Bee
The LearnEnglish Team

Hello Jeremy! Could you please explain the use of "need" as a model verb? I've been through the examples given by OALD  and it's kinda confusing. Never in my life have I ever found a grammar rule so confusing. Please explain when exactly we're not required to put 'to' after "need"     Here's the examples:
     "I need hardly tell you that the job is dangerous"
      "If she wants anything, she need only ask" here "need" without 's' is creating problem.
      "All you need bring are sheets"
Here in my part of the world, teachers who teach B.A English, have been telling the students not to put "to" after need. I was surprised to see that every grammar book gives only this one use of "need" and forgetting or denying its use with to.