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が必要・必要がある
Means "needed" or "necessary". が{必要|ひつよう} is used mainly with nouns (can be used with verbs by appending こと before). {必要|ひつよう}がある is used with verbs.
[!info] Example {今時|いまどき}、{携帯電話|けいたいでんわ}==が必要です==。 Nowadays, a cell phone is necessary.
今日はパン{屋|や}に行く==必要がない==。 No need to go to the bakery today.
わけにはいかない
Used with verbs for a meaning of "cannot do" (which cannot be your inability, but an external factor). Used with verbs in negative form (ない) for a meaning of "cannot be avoided". Can be used as わけはいかない or わけにはいきません.
[!info] Example {今日|きょう}の{弟|おとうと}の{誕生日|たんじょうび}に行か==ないわけにはいかない==だから、{飲|の}み{会|かい}に行かない。 There's no way I don't go to my younger brother's birthday today, so I won't go to the afterwork.
{本当|ほんとう}に飲み会に行く==わけにはいかない==よ。 There's really no way I can go to the afterwork.
ずにはいられない
Means "can't help but do something". Made with a verb in the negative form (ない) without the ない ending and adding ずにはいられない.
[!info] Example {悲|かな}しいのでアイスクリーム食べ==ずには入れれない==。 I'm sad so I can't help but eat ice cream.
こと as "should do"
V (dictionary)
V (ない)
+ こと
Means "should do". Mostly used to set rules.
[!info] Example ここに{車|くるま}を{止|と}めない==こと==です。 Don't park your car here.
ことはない
This has two meanings:
- it is not necessary to
- it never happens
Used with a verb in the dictionary form.
[!info] Example {走|はし}る==ことはない==。まだ{時間|じかん}がある。 There's no need to run. There's still time.
ことになっている
This form is used to express rules or expectations. Used with dictionary (る) and negative (ない) form of verbs.
[!info] Examples {指|ゆび}で食べない==ことになっている==。 You're not supposed to eat with your fingers.
Must
なくちゃ・なきゃ
Only used on casual spoken conversations. This form is used with the negative form of a verb (ない), and adding なくちゃ and optionally one of the following ending: だめ, いけない, いけません, ならない, なりません.
なくちゃ is the shortened form of なくては. なきゃ is the shortened form of なければ.
[!info] Example {歯|は}を{磨|みが}か==なくちゃいけない==。 I must brush my teeth.
なくては「いけない・ならない」
Also formed with the negative form of a verb.
[!info] Example 今{遅|おそ}いね。{寝|ね}に行か==なくてはいけない==。 It's late now. I have to go to sleep.
なければ「いけない・ならない」
Also formed with the negative form of a verb.
[!info] Example {事務所|じむしょ}では{静|しず}かにし==なければならない==。 You have to be quiet in the office.
ないといけない
Also formed with the negative form of a verb. In casual contexts, can be shortened as ないと.
[!info] Example 今日は{勉強|べんきょう}し==ないといけない==。 I have to study today.
べきだ
Means "should do", "must do". Used with verbs. する can be used as するべき or すべき.
[!info] Example {来週|らいしゅう}{試験|しけん}があるので{勉強|べんきょう}す==べきです==。 There is an exam next week, so I should study.
てからでないと
Means "must first do". The second part becomes possible after the first one. Used with a verb in て form, with からでないと or からでないければ.
[!info] Example {払|はら}==ってからでないと=={入|はい}るができること。 You can enter only after paying.
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Comparing いけない and ならない
- ==いけない== is the standardly used form. It implies a subjective view, that it would be better to do something, to avoid negative consequences (eg: brushing one's teeth, eating well, etc).
- ==ならない== is usually more formal or in written form. It implies a is more objective view, something that has to be done because there's no other way (eg: respecting the law, paying taxes, etc).
Comparing なくては, なければ and ないと
なくては and なければ are generally interchangeable in speech. However:
- ==なくては== is generally used when there is no penalty for not complying.
- ==なければ== is generally used when there is a penalty for not complying.
- ==ないと== is lighter and does not imply that there can or cannot be penalty. Its construction is based on the Conditional#と particle, and should be based on knowledge or experience.
Must not
「ちゃ・じゃ」「だめ・いけない・いけません」
Only used on casual spoken conversations. いけません is a bit more formal. This form is used with a V-て, depending on the ending:
- with て, ちゃ is used.
- with で, じゃ is used.
[!info] Examples その{花|はな}を食べ==ちゃだめ==。 Don't eat those flowers.
{花瓶|か|びん}の水を{飲|の}ん==じゃいけない==。 Don't drink the water of the flower vase.
てはいけない
Used in a more formal or written manner. Used with a V-て.
[!info] Example {鉄道|てつ|どう}に{歩|ある}い==てはいけません==。 Do not walk on the railroad.
なくてもいい
Means "don't have to". Used with a verb in the negative form (ない), appending なくてもいい. Can also be used as なくていい.
[!info] Example 今日はレストランで{手伝|て|つ}わ==なくてもいい==。 You don't have to help out at the restaurant today (it's okay if you don't help at the restaurant today).
べきではない・べきじゃない
The contrary of べきだ is べきではない・べきじゃない.
[!info] Example {泥|どろ}を食べる==べきではない==。 You should not eat mud.
ても構わない
V (て)
い-Adj い くて
な-Adj + で
N + で
+ もかまわない・もかまいません
Means "it doesn't matter if", "I don't mind".
[!info] Example ここで{座|すわ}==っても構いません==か。 Would it be okay if I sit here?
==かまわない==よ。 I don't mind.
TODO
ないと
must do; unless/if you don't ~
https://jlptsensei.com/learn-japanese-grammar/%e3%81%aa%e3%81%84%e3%81%a8-naito-meaning/ https://bunpro.jp/grammar_points/%E3%81%AA%E3%81%84%E3%81%A8