Much like てはいけない
states that something ‘must not’ be done. However, this is where the similarities end. なくてはいけない
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. いけない
simply means ‘cannot go’, and comes from the negative potential form
of 行（い）く. 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 いけない