a conditional 'if' (attached to the て-form of verbs): expresses 'if [someone] does ...' often linked to negative or result clauses.
そんなことをしては困るよ。
You mustn't do something like that (if you do that, it'll be a problem).
忘れては困るから、メモしておいてください。
Please write it down because it would be a problem if you forgot.
遅刻しては評価が下がるかもしれない。
If you're late, your evaluation might drop.
used like 'since / if you are going to ...' — presupposing that an action will be taken and drawing a conclusion or comment from that premise.
これ以上無駄遣いをしては、貯金なんてできないよ。
If you keep wasting money like this, you won't be able to save anything.
そんなに遅くまで働いては体を壊すよ。
If you work that late, you'll ruin your health.
出かけては帰ってこないなら、連絡くらいしてほしい。
If you're going out and not coming back, I wish you'd at least get in touch.
indicates repeated or alternating actions (one after another; '... and then ... repeatedly').
彼はテレビを見てはため息をつく。
He watches TV and sighs again and again.
その店は開いては閉め、また開いては閉めている。
The shop keeps opening and closing again and again.
can add emphasis or a particular tone in set phrases (e.g., in expressions like 言っては何だが ... 'I don't mean to say this, but ...').
言っては何だが、君の説明は少し分かりにくい。
I don't mean to be harsh, but your explanation is a bit hard to follow.
親切にするのはいいが、してはしては相手が甘えるだけだ。
It's fine to be kind, but if you keep doing everything for them they'll just become spoiled.