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しなくては

must do; have to do

→ 為る
phrase Very common #752
must do; have to do Often followed by いけない or ならない. On its own, it can also imply that something must be done, with the rest of the expression left unstated.
JA: 今日は勉強しなくてはならない。
EN: I have to study today.

今日はたくさんしなくてはいけないことがあります。

I have a lot to do today.

学校で立派にやっていくつもりなら、頑張って勉強しなくては。

If you're going to do well in school, you'll have to study hard.

人のオフィスに入るときは、まずノックしなくては。

When entering someone else's office, you should knock first.

屋根裏部屋のネズミを退治しなくてはと彼はいっている。

He says we have to get rid of the rats in the attic.

落ち着いて勉強しなくてはだめ。

You need to study calmly.