Difference between "なければなりません", "なくてはいけません" and "が必要です”？

[forgottenmems]

I know there must be a difference between "なければなりません", "なくてはいけません" and "が必要です" but I don't know what it is. I was under the assumption that they meant, "I need to [blank]" or "I have to [blank]" but I'm not really sure in what contexts you use them. Can they be used interchangeably with any verb/noun + koto?/or even just a noun in some cases (ga hitsuyou desu)? If anyone knows, I'd appreciate a response. Thank you

 

[Half-n-Half]

As far as I understand it, in general,
～ないとだめ/いけない/ならない
～なければだめ/いけない/ならない
～なくてはだめ/いけない/ならない

are, for all intents and purposes, the same. It's just a matter of preference and you'll be understood no matter which you use. You can also shorten it to なくちゃ(だめ/いけない/ならない) or なきゃ depending on how familiar you are with the person you are speaking with. I even remember one of my Japanese lab mates telling me that sometimes he might say something like 行かなきゃいけません to a professor or a 目上, but I think you better ask Toritoribe to be sure on that.

Also, が必要です sounds more formal to me.

 

[Toritoribe]

As HnH-san wrote, the difference between ならない and いけない is subtle and they are interchangeable in most cases. The following thread might be somewhat helpful.

Have to | Japan Forum

There is another form for "to need to do"; "casual form of a verb" + 必要がある.

 

[forgottenmems]

Thanks Guys The other thread helped a lot too However, I have one more question. What about "I don't need to [blank]"? I'm lost on how to properly express that sentiment in Japanese. Can you use the same endings (nakereba narimasen etc.) reconfigured or do you have to use different verb endings?

 

[Toritoribe]

"casual form of a verb" + 必要が/はありません
～なくてもいいです

 

[Half-n-Half]

Could you also use "casual form of a verb" + ことはありません ?
For example,
謝ることはないんですよ。

 

[forgottenmems]

"casual form of a verb" + ことはありません ?
For example,
謝ることはないんですよ。

Got it. Thanks

 

[Toritoribe]

Could you also use "casual form of a verb" + ことはありません ?
For example,
謝ることはないんですよ。

Yeah, that's OK, too.

無理することはない。