Need is a strange verb. You need a to in the indicative (You need to go). Except when you don't. In the present, you can say You need only go to Manchester. Perhaps it's because there's something between the need and the infinitive.
Need with a noun object: I need water for my roses. You need time to think about that. We all need friends.
Need with a verb object, positive sentences: You need to ask again. They need to be careful. (Meaning sometimes very close to should.)
Need with a verb object, negative sentences: You do not/don't need to ask again. They do not/don't need to be careful. Need as a modal, negative sentences, colloquial option: You needn't ask again. (Meaning sometimes very close to mustn't.)
Need with a verb object, questions: Do you need to ask? Need you ask? (Also Do you need ask? though this is rarer)
How to distinguish between them? Why do you need to? Just keep to the above usage."
Perhaps, on reflection, I should have said you don't have a to when there's something between the need and the infinitive. The something can be a not or an only etc.
There seems to be a difference between needs and need in the 3rd person singular present. He needs to go to Manchester. He need only go to Manchester.
Need as a modal verb works like other modal verbs such as can: there's no third person singular -s, and you make the interrogative by inversion (need you?) and the negative by simply adding "not" (you needn't).
Need as an ordinary lexical verb works like other lexical verbs such as want: there's a third person singular -s, and you make the interrogative and negative using "do" (do you need? you don't need).