Both these expressions are used when giving permission or advice for someone not to do something in the future. The meaning is similar to "It is not necessary for you to do it"/"There is no need for you to do it".

Here are some typical example sentences:

You needn't/don't need to buy eggs. We have enough.
(= It is not necessary to buy eggs.)
You needn't/don't need to take your umbrella. It won"t rain.
(= It is not necessary to take your umbrella.)

The two expressions mean the same thing. However, if I am being careful in either writing or speech, I will use "don't need to" when making general statements:

You don't need to be intelligent to learn a foreign language.
（外国語を学ぶのに頭の良さは必要ない）
Children don't need to pay full fare if they are under 14 years old.
（子供は14歳以下なら全額払わなくてもよい）

In the next Column I will take up another point connected with "need".

The first of these two expressions looks like a present perfect form (because of the "have + past participle"). However, it is a past form, just like the second expression.

The difference in meaning is clear. "I needn't have done it" means "It did it, but it was not necessary". This expression can be used, for example, when we waste our time or energy doing something:

I needn't have bought butter. There is still a bit in the fridge.
（バターは買わなくてもよかったのに。まだ冷蔵庫に少しあるから）

This means that I bought the butter, but it was not necessary to do so. Here is another example:

I needn't have taken my umbrella, since it didn't rain.
（雨が降らなかったから、傘は持って行かなくてもよかったんだが）

This means I took my umbrella, but the umbrella was not necessary, since it didn't rain.

The meaning of the second expression is "It was not necessary to do it, and I did not do it". We can say, for example:

I didn't need to buy butter. We have a lot in the fridge.
（バターは買わないですんだ。冷蔵庫にたくさんあるし）
I didn't need to take my umbrella, since I knew it was not going to rain.
（雨が降りそうにないのは知っていたから、傘は持って行かずにすませた）

It is certainly not everyone who makes this distinction. The tendency in modern English is to use "didn't need to do it" for both kinds of situations.

If John returns home one night and finds that money has been stolen from his house, he might say, "It had to have been a burglar" (泥棒のしわざにちがいない). This means that he is almost certain that the person who stole the money was a burglar.

This is an expression which we use when we make strong assumptions about what happened in the past. For example, if the police are examining a building that has been destroyed by fire, they may say:

The fire spread very quickly. It had to have been arson.
(火の手は非常に速かった。放火（だった）にちがいない)

A less emphatic way of saying this is to use "must have been", which, in this sense, can also be used:

It must have been a burglar.
It must have been arson.

Here are some other sentences using "had to have/must have + past participle to mean assumption:

John passed the exam. He had to have/must have studied hard.
(ジョンは試験に合格した。彼は一生懸命勉強したにちがいない)
Her English was perfect. She had to have/must have lived abroad.
(彼女の英語は完ぺきだ。留学経験があるにちがいない)

The thing to notice about all these expressions is that although there is a "have been" in the sentence, the meaning is past, and not present perfect (see Column no. 018).

As a final point, I should mention that a computer search of a very large corpus shows that "must have been" is about 1000 times more common than "had to have been".

This is only natural, since the less emphatic way of saying something tends to be the more common one in everyday life.

In the next Column I will look at "had to have + past participle" once more.

In the previous Column, I dealt with this pattern when it is used to show an assumption. In this Column, I will give some examples of when it is used to show necessity.

Here are some example sentences, followed by paraphrases which show this necessity:

You had to have lived in the village for years and years before they accepted you as one of them.
(村の一員として受け入れられるまでには長年その村に暮らさなくてはならなかった)
＝ It was necessary for you to have lived in the village for years and years ...
You had to have experienced the war in order to understand how terrible it was.
(その戦争がどれほど恐ろしいものかを理解するには、それを経験しなくてはならかなった)
＝ It was necessary for you to have experienced the war in order to …
You had to have been a resident for ten years before you could apply for citizenship.
(市民権の申請ができるまでには10年間居留している必要があった)
＝It was necessary for you to have been a resident for ten years before ...

This pattern is rather a complicated one, and gets more complicated if we use it in the passive form:

All the goods had to have been paid for by the customers before they were allowed to take them out of the shop.
(すべての商品は、代金が払われてはじめて、店から持ち出すことができた)
＝ It was necessary that all the goods were paid for by the customers before ...

Note that in this use of "had to have + past participle" we cannot use "must have + past participle" with this meaning, as we can in the case of assumption (see Column 019).