ちゃいけない expression


> やっちゃいけないことをやっちゃった。

I noticed that it uses 2 verbs. One is `遣る` and the other one is `ちゃう`. They
can stand alone as an individual verbs or they can combined together as in the
example sentence.

The way I make sense of `やっちゃいけない` is that it combines `遣る` and `ちゃう` to
create a command telling someone to not do something. But, I don't understand
how to make sense of `やっちゃった`. It still uses the same 2 verbs but when
combined, they don't indicate a command and `ちゃった` is just a past tense of
`ちゃう`, so why combine them into `やっちゃった` then?




ちゃう is not a verb that can stand on its own, as far as I know, except in some
dialects as a form of 違う.

Are you familiar with ～てしまう? This form, in this case, means the result was
undesirable. The 過去形 being った is normal for godan verbs ending in う. It can
contract to ちゃう.

Superficially やっちゃいけない may resemble it somewhat, but this やっちゃ is やっては.
～ては（駄目・いけない・ならない）is a form meaning 'must not', 'should not', etc. Are you
familiar with this?

Accordingly, the sentence is something like 'I did something I shouldn't
have.'




ちゃう is not a verb -- やっちゃいけないこと is simply a reduction of やってはいけないこと "Something
that must not be done". Your understanding of やっちゃいけない is not correct, as it
is not a command in this context at all.

Accordingly the sentence simply translates to:

> やっちゃいけないことをやっちゃった  
>  I ended up doing something I wasn't meant to do.

