Modal Verbs: Need - Used to

Need

To require sth/sb because they are essential or very important, not just because you would like to have them: [vn] Do you need any help? It's here if you need it. Don't go - I might need you. They badly needed a change. Food aid is urgently needed. What do you need your own computer for? You can use ours. I don't need your comments, thank you. [V to INF] I need to get some sleep. He needs to win this game to stay in the match. You don't need to leave yet, do you? [V _Ing, v to INF] This shirt needs washing. This shirt needs to be washed.
[V to Inf] used to show what you should or have to do: All you need to do is complete this form. I didn't need to go to the bank after all - Mary lent me the money.

Idioms
Need (to have) your head examined: (Informal) to be crazy
(Negative need not, short form needn't (BrE) need (not) do sth | need (not) have done sth used to state that sth is/was not necessary or that only very little is/was necessary; used to ask if sth is/was necessary: You needn't finish that work today. You needn't have hurried (= it was not necessary for you to hurry, but you did). I need hardly tell you (= you must already know) that the work is dangerous. If she wants anything, she need only ask. All you need bring are sheets. Need you have paid so much?

Grammar
In BrE there are two separate verbs need: Need as a main verb has the question form do you need? The negative you don't need and the past forms needed, did you need? And didn't need. It has two meanings:

• To require something or to think that something is necessary: Do you need any help? I needed to get some sleep.
• To have to or to be obliged to do sth: Will we need to show our passports?

Need as a modal verb has: need for all forms of the present tense, need you? As the question form and need not (needn't) as the negative. The past is need have, needn't have. It is used to say that something is or is not necessary: Need I pay the whole amount now?
In NAmE only the main verb is used. This leads to some important differences in the use and meaning of need in British and American English.
In NAmE it is more common for need to be used to speak about what is necessary, rather than about what you must do: I don't need to go home yet - it's still early. (BrE and NAmE = it isn't necessary) You don't need to go home yet - we never go to bed before midnight. (BrE = you don't have to.)
The difference is even more noticeable in the past tenses: He didn't need to go to hospital, but he went just to reassure himself. (NAmE) He needn't have gone to hospital, but he went just to reassure himself. (BrE = he did something that wasn't necessary.) He didn't need to go to hospital after all - he only had a few bruises. (BrE= he didn't go.)

Used to

Used to say that sth happened continuously or frequently during a period in the past: I used to live in London. We used to go sailing on the lake in summer I didn't use to like him much when we were at school. You used to see a lot of her, didn't you?

Used to / be used to
You use used to do sth to talk about something that happened regularly or was the case in the past, but is not now: I used to smoke, but I gave up a couple of years ago.
You use be used to sth / to doing sth to talk about something that you are familiar with so that it no longer seems new or strange to you: We're used to the noise from the traffic now. I'm used to getting up early. You can also use get used to sth: Don't worry - you'll soon get used to his sense of humour. I didn't think I could ever get used to living in a big city after living in the country.

Grammar
Except in negatives and questions, the correct form is used to: I used to go there every Saturday. I use to go there every Saturday.
To form questions, use did: Did she use to have long hair? Note that the correct spelling is use to, not "used to". The negative form is usually "didn't use to", but in BrE this is quite Informal and is not usually used in writing. The negative form used not to (rather formal) and the question form used you to...? (Old-fashioned and very formal) are only used in BrE, usually in writing.