A2
なければいけない — must
How Japanese expresses obligation with なければいけない and なくてはいけない, the contractions used in speech, and the difference between いけない and ならない.
There is no Japanese word for must. Obligation is built out of a negative condition: literally if I do not go, it will not do.
行かなければいけません。
Ikanakereba ikemasen.
I have to go.
薬を飲まなければいけません。
Kusuri o nomanakereba ikemasen.
I have to take my medicine.
Take the plain negative, drop ない, add なければ — 行かない → 行かなければ — then close with いけない or いけません.
Two halves, several combinations
The first half can be なければ or なくては; the second can be いけない or ならない. All four combinations occur and mean the same thing. ならない is more formal and more common in writing and rules; いけない is what people say.
ここで靴を脱がなくてはいけません。
Koko de kutsu o nuganakute wa ikemasen.
You have to take your shoes off here.
In speech it collapses
なきゃ and なくちゃ are the contractions, and the ending is often dropped altogether — the listener supplies it.
もう行かなきゃ。
Mō ikanakya.
I’ve got to go.
That is a complete, natural sentence with the it won’t do left off.
The mistake is dropping the negative
行けばいけません。
行かなければいけません。
The condition has to be negative. 行けば means if I go, so 行けばいけません says roughly if I go, that’s no good — the opposite of the intended meaning.
Not having to
The negative of the obligation is a different construction: なくてもいい, it’s all right even if you don’t. See permission.
明日は来なくてもいいです。
Ashita wa konakute mo ii desu.
You don’t have to come tomorrow.