must/need [必要]{ひつよう}がある vs. なくてはいけない and [必要]{ひつよう}だ vs. [要]{い}る


This is a two part question, both pertaining to expressions involving "必要".

  1. What is the difference between 必要がある and -なくてはいけない/ -なければならない? The difference between the latter two expressions is explained in [this](https://japanese.stackexchange.com/questions/2500/must-do-%EF%BD%9E%E3%81%AA%E3%81%91%E3%82%8C%E3%81%B0%E3%81%AA%E3%82%89%E3%81%AA%E3%81%84-vs-%EF%BD%9E%E3%81%AA%E3%81%8F%E3%81%A6%E3%81%AF%E3%81%84%E3%81%91%E3%81%AA%E3%81%84) question, but I'm still unclear as to where "[必要]{ひつよう}がある" comes into play (e.g. if it is synonymous with one or both of those, or if it carries a different nuance altogether).

> I need to do my homework.  
> 宿題をしなくてはいけません。  
> 宿題をする必要があります。

  2. What is the difference between (noun)が[必要]{ひつよう}です and (noun)が[要]{い}ります? Unlike the expressions in question number one, these expressions are used when expressing the necessity of an object or thing.

> I need a new motherboard.  
> 新しいマザーが必要です。  
> 新しいマザーが要ります。




While both ～必要がある and ～なくてはいけない are used to express that something is
necessary or must be done, they are both used slightly differently.

Take for example the following sentances:

> コンピュータを再起動する必要があります。  
>  You need to restart the computer.

> コンピュータを再起動しなくてはいけません。  
>  You/I/etc must restart the computer.

In the above sentances `必要があります` is less personal and has a more instructional
tone to it. The message is actually what my Mac displays when it wants me to
restart. In this scenario `しなくてはいけない` wouldn't be appropriate, as it would
sound demanding and impolite.

`しなくてはいけない` has a more colloquial, personal tone to it, while `必要がある`
emphasises the degree of requirement, and is more formal/polite. For example
here is a sentence taken from the terms and conditions of a form I am
currently filling in:

> 申請者ご本人を確認する必要があります。  
>  We are required to verify the applicants identity.

Again, `しなくてはいけない` wouldn't really work in this more formal written scenario.

Finally, if you take the examples posted by @yadokari above:

> 宿題をする必要があります  
> It is necessary that i do homework

> 宿題をしなくてはいけません。  
> I have to do homework

While both are fine, I doubt you'd hear many kids running around saying
`宿題をする必要があります`, they would almost certainly use a form of `宿題をしなくてはいけません`.

As for the second part of your question, looking up the words in the
dictionary gives a pretty good explanation.

[code]

     必要［名・形動］なくてはならないこと。どうしてもしなければならないこと。
        - Must not be without, Must be done / is necessary.
[/code]

[code]

     要る［動ラ五（四）］費用・品物・時間などが必要になる。入用である。
        - Is needed, becomes necessary, mainly used with products, goods, time, money, etc.
[/code]

The meaning of both 要る and 必要 are again very similar, as above there is a
slight difference in the levels of politeness/formality. I would also say that
要る has a lesser degree of need than 必要 does.

For example:

> 新しいパソコンが要ります  
>  I need a new computer.

The nuance of the above sentence would be that while your saying that you need
a new computer, it may not be a requirement (Maybe your current computer works
just fine, but it runs slow, and it would just be great if you had a new one).

While if you used ～必要です in the same sentence, it adds a greater degree of need
to the statement, the computer becomes a requirement that you cannot do
without (Maybe your computer is too slow to do what you want to do, or maybe
it's broken, but the bottom line is that you must have a new computer to
accomplish whatever it is you want to do).




While both ～必要がある and ～なくてはいけない are used to express that something is
necessary or must be done, they are both used slightly differently.

Take for example the following sentances:

> コンピュータを再起動する必要があります。  
>  You need to restart the computer.

> コンピュータを再起動しなくてはいけません。  
>  You/I/etc must restart the computer.

In the above sentances `必要があります` is less personal and has a more instructional
tone to it. The message is actually what my Mac displays when it wants me to
restart. In this scenario `しなくてはいけない` wouldn't be appropriate, as it would
sound demanding and impolite.

`しなくてはいけない` has a more colloquial, personal tone to it, while `必要がある`
emphasises the degree of requirement, and is more formal/polite. For example
here is a sentence taken from the terms and conditions of a form I am
currently filling in:

> 申請者ご本人を確認する必要があります。  
>  We are required to verify the applicants identity.

Again, `しなくてはいけない` wouldn't really work in this more formal written scenario.

Finally, if you take the examples posted by @yadokari above:

> 宿題をする必要があります  
> It is necessary that i do homework

> 宿題をしなくてはいけません。  
> I have to do homework

While both are fine, I doubt you'd hear many kids running around saying
`宿題をする必要があります`, they would almost certainly use a form of `宿題をしなくてはいけません`.

As for the second part of your question, looking up the words in the
dictionary gives a pretty good explanation.

[code]

     必要［名・形動］なくてはならないこと。どうしてもしなければならないこと。
        - Must not be without, Must be done / is necessary.
[/code]

[code]

     要る［動ラ五（四）］費用・品物・時間などが必要になる。入用である。
        - Is needed, becomes necessary, mainly used with products, goods, time, money, etc.
[/code]

The meaning of both 要る and 必要 are again very similar, as above there is a
slight difference in the levels of politeness/formality. I would also say that
要る has a lesser degree of need than 必要 does.

For example:

> 新しいパソコンが要ります  
>  I need a new computer.

The nuance of the above sentence would be that while your saying that you need
a new computer, it may not be a requirement (Maybe your current computer works
just fine, but it runs slow, and it would just be great if you had a new one).

While if you used ～必要です in the same sentence, it adds a greater degree of need
to the statement, the computer becomes a requirement that you cannot do
without (Maybe your computer is too slow to do what you want to do, or maybe
it's broken, but the bottom line is that you must have a new computer to
accomplish whatever it is you want to do).

