Much like
てはならない,
なくてはならない states that something ‘must not’ be done. However, this is where the similarities end.
なくてはならない is an example of a double negative in Japanese, and actually has the meaning ‘(A) must be done’, or ‘must do (A)’.
While this grammar point is usually translated as ‘must do’, the literal translation is ‘must not, not do (A)’. This double ‘not’ is where many learners have difficulties.
ならない simply means ‘cannot become’, and comes from the negative form of
なる.
To use this structure, simply create the negative form of a verb, and then convert ない to なく (the conjunctive form of an
い-Adjective), followed by
て. After this,
は is added, and then finally
ならない.
-
私（わたし）は次（つぎ）のバス停（てい）で降（お）りなくてはならないです。
I have to get off at the next bus stop. (I must not, not do it)
-
トムはタカにお金（かね）を返（かえ）さなくてはならないです。
Tom must give back money to Taka. (He must not, not do it)
-
今日（きょう）は漢字（かんじ）の勉強（べんきょう）をしなくてはならないです。
I have to study kanji today. (I must not, not do it)
ならない is considered to be the most formal structure out of
ならない,
いけない, and だめ. It is regularly used in formal writing, or semi-formal/formal speech.
いけない (the most common variation) and だめ (the most casual variation) are also acceptable in many situations.
The ては in なくては is often shortened to ちゃ, as this is easier to say.
なくちゃ may then be followed by
ならない. However, due to
ならない being quite formal,
なくちゃ (a casual structure) will not be used with
ならない on a regular basis. It is more common to see
なくちゃ paired with
いけない, or だめ.
-
今日（きょう）は沢山（たくさん）走（はし）ったからシャワーを浴（あ）びなくちゃならない。
Because I ran a lot today, I must take a shower.
-
バーベキューに肉（にく）を持（も）って行（い）かなくちゃいけない。
I gotta bring meat to the barbeque.
-
冬（ふゆ）になる前（まえ）にキャンプに行（い）かなくちゃだめだね。
We gotta go camping before it becomes winter, right?
These casual structures are very similar to words like ‘gotta’ in English.