How to "reverse" 〜てから〜てはなりません?


Consider the sentence:

> シャワーを浴び **てから** 、プールに入らなく **てはなりません** 。

As far as I understand, てから here is used to glue them into “You must go to the
pool after taking a shower” which makes the most sense.

However, I was confused at first and thought it meant “After taking a shower,
you must go to the pool [and nowhere else]”, which doesn't make a lot of sense
in real life.

What would be the grammatical way to “reverse” the sentence? Would it be
something like this?

> After you finish your work, you must go home [and do nothing else which is
> not going home].
>
> 仕事が終わりしまったら、家に帰らなくてはなりません。

> You must report to me after finishing your work.
>
> 仕事を終わってから、私に報告しなくてはなりません。




They are to some extent ambiguous, just as (I think) _You must go to the pool
after taking a shower_ is. I don't think there's any grammatical reason that
it can't mean _After taking a shower, you must go to the pool [and nowhere
else]_. As you say, it is unlikely, but it is a matter of meaning.

Practically, you also got the answer already. Basically:

  * A(し)てから、B(し)なくてはならない : You need to do A first before doing B.

It is like (AしてからBする) + なくてはならない = _It must be 'do A, then do B'_.

The 'reverse' version is just use たら=if

  * A(し)たらB(し)なくてはならない = If you do A, you must do B.

So, with the correct conjugation,

  * 仕事が終わったら、家に帰らなくてはなりません。

This means, as you intend, _After finishing the work, you must go home (you
can't stay in the office)_.

* * *

These things said, one example I came up with is:

  * 仕事が終わってから、子供を迎えに行かなくてはなりません

This can be 仕事が終わったら, but I think both are fine and usually both are
interpreted as _After work, I must go pick up my kid_. Ultimately it depends
on the meaning.

