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 skinnypigeon,
"Need' is sometimes described as a semi-modal verb - that is, a verb which can function both as a modal and as a regular/normal verb.  When we use it as a modal, its form is similar to other modals, meaning we use a bare infinitive (without 'to`), form negatives by adding 'not', questions by inversion and a perfective form by adding 'have', just as we do with 'should', for example:
Need we go now?
We needn't go yet.
However, the verb is slowly changing from a modal to a regular verb and so the forms above are, gradually, becoming less common and are starting to sound a little archaic.  In fact, the positive form is no longer used in modern English and you can only really see the modal form of 'need' in questions and negatives.  Even with these forms it is much more common these days to use 'need' as a regular verb, similar to 'want':
Do we need to go now?
We don't need to go yet.
Obviously I can't comment on your textbooks or teachers in your country but do remember that English exists in many different standard forms in many countries and what is archaic in, for example, British English may be quite normal in standard English in other countries.
I hope that clarifies it for you.
Best wishes,
The LearnEnglish Team

Thanks very much indeed Peter! I really appreciate that. Your explanation certainly does clarify.

Hello , I would like to know , what is the correct form , to write Objectives in English?. I'm confused , when I want to write an objective. I don't know in which verb add "to" or omit it . Sorry If I have mistakes. Please !

Hello Daniela R,
I'm sorry, but I'm not sure I understand your question.  Do you mean how do we express the reason (purpose) of our actions using infinitives?  If so, then we use 'to'.  For example:
I went to the shop to buy some fruit.
[buying some fruit was the reason for going to the shop]
I hope this answers your question.  If you had something else in mind, please reply and include an example sentence, and we'll try to help.
Best wishes,
The LearnEnglish Team

Dear Sir,
Please help me to understand the usage of ' keen to &  keen on ' why because I wrote a sentence in my CV with keen to ( keen to work with a dynamic organization...............). 

'Keen to' is used to talk about something you want to do but have not yet done:
'I'm very keen to meet him.' [= I haven't met him yet, but I want to]
'Keen on' is used to talk about something that you like very much right now:
'I'm really keen on cooking.' [= I enjoy cooking]
The sentence you quote from your CV looks fine as it describes your hopes and goals in the future.
I hope that clarifies it for you.
Best wishes,
The LearnEnglish Team

Hello Peter,
Thank you very much for your prompt reply.  

Dear grammar experts,
Should we use second TO if two infinities connected with AND or OR?
Please, write back as soon as possible.
Alyona, Kazakhstan

Hi Alyona,
I suppose you are asking about a sentence such as:
He would like to shake the president's hand and to speak with her.
That is correct, though people also say:
He would like to shake the president's hand and speak with her.
If I haven't answered your question, please give an example of what you mean so that I can help you better.
Best wishes,
The LearnEnglish Team

Dear sir,
"he wept to see the desolation caused by the flood"
how 'to see' is expressing cause? I interpret it as purpose not cause.