To need and need

   I need to speak to your manager urgently - can you put me through?
 

Need is a modal auxiliary verb.

To need has a similar function and meaning and is more commonly used.  

Note that  

1.     Like other modal auxiliaries, need has no infinitive and we use the same form with 'I', 'you', 'he', 'she', 'it', 'we' and 'they'. 'Do' is not used to form negative or interrogative sentences.

2.     To need takes the usual forms depending on the subject and the tense.

3.     To need can also be followed by a noun or gerund.

You need a visa to visit Japan.

My apartment needs cleaning.

This is not possible with the modal auxiliary need.

You don't need a visa to visit Japan.   

You needn't a visa to visit Japan.             

To need and need are both used to talk about something that is necessary.

Need cannot be used in the affirmative form instead of to need but we might read or hear it very rarely in this kind of sentence:

            He need have no fear of losing her - she loves him very much.

Similarly, we can use it with negative or restrictive words such as 'hardly', 'never', 'nobody' or 'only'.

            We need only provide lunch for the visitors - they'll have dinner at the hotel.

            I need hardly say how important it is that we respect the deadline.

            If we solve this before the end of the day, he need never know about it.

Need can be used in the negative form instead of to need.

You don't need to come to the meeting if you don't want to.

=

You needn't come to the meeting if you don't want to.

Most of the time, either form can be used but don't need is usually preferred for a general rather than a specific necessity:

            You needn't/don't need to email him today - it can wait until tomorrow.

            You don't need to be fluent in English to write an email.

Needn't/don't need are mainly used in the same way as 'don't have to'. 'Don't have to' is sometimes preferred when the lack of necessity is a general truth or decided by someone other than the speaker:

            You needn't/don't need to/don't have to bring any wine tonight - we have plenty.

            You don't have to take wine to Lionel's parties - he always has plenty.

To need is preferred in modern, spoken English:

Do you need to leave now?

Need is considered very formal:

Need you leave now?
 

These are very similar and often interchangeable but to need sometimes suggests there is a choice or a potential personal benefit:

            I need to send three more emails before I go home tonight.

            I need to buy some bread on the way home - is there a baker's near here?

whereas have to can suggest a lack of choice or something decided or imposed by another person. There is also a suggestion of urgency a negative consequence if something is not done:

            I have to finish this today because I'm going on holiday tomorrow.

 The boss told me I have to go to Paris next week - he wants me to speak to the client

 personally.

            In the past simple, 'had to' is used more than needed to.

'Need' is also a noun with a related meaning. It can be used in different ways. For example:

There's no need for another office in Paris - we already have two.

There's no need to finish that today - I'll do it tomorrow.

We have need of some new computers.

I'm in need of chocolate.

The company respects the needs of its overseas staff.

    I need to finish this by Friday.

     Nobody need find out if we keep this to ourselves. 

   She doesn't need to work today.

    You needn't worry - I can fix it.

   Shall we go for a coffee or do you need to get back to the office?

   Need we make a decision now or can we wait till next week?

   It needn't be expensive to visit London - buses are cheap and most museums are free.

   My office needs tidying before my visitors arrive.

  You need to be getting much better results than this.

     We'll need to look at these figures more closely later to develop a strategy.