How to "reverse" 〜てから〜てはなりません?

Question

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。

Answer 1

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).

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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.