Is negative + て + いけません a natural way to say "must do"?

After teaching the 〜て form, my grammar book provides 〜てはいけません as one of its usages. It shows examples such as this one:

私は今日3キロ走らなくてはいけません。
I have to run 3 km today.
If I understand it literally, it says “Not running 3 km today won’t do”.

Is it a natural way of forming a sentence, a negation on top of a negation? Do people really talk this way, or is it just a trick that the book is employing to bypass grammatical structures we haven’t studied yet?

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It’s the typical way yeah. I mean there are other ways, しないとだめ、しなければならない、しなくちゃ, etc

But all of them are double negatives or a single negative with the second one omitted

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This is normal though it is usually abbreviated as しなきゃ (or similar). There are many other ways to express this same meaning, but you will find as you immerse that it is very common.

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I know you mean this, but for the sake of the question asker, I would add omitted but implied.

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Yah, you’ll see the construction all the time in native materials. Often abbreviated, mind:

走らなくてはいけない → 走らなくちゃ
走らなければいけない → 走らなきゃ
走らないといけない → 走らないと

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There are solely affirmative ways of saying “must do” something, but there’s nothing awkward or contrived about the format you mentioned. This is probably the most natural way to do it.

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Like べき, for example.

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べき is different than the meaning of what he’s asking about tho

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That’s just down to must’s flexibility though. ~べき does count as a “natural way to say ‘must do’” that isn’t a double negative.

I was imagining a more literal, (slightly) less idiomatic construction, though.

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I’m not sure if people would read belthazars post and realize that saying 勉強すべき instead of 勉強しなきゃ for “I gotta study” is a mistake, or that belthazar himself even realizes. Just wanted to point out that while you can map them to one English word, they’re not the same…

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Sure, it’s worth clarifying.

べき

すべき is fine

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Now I’m really not sure you realize べき and the double negative things aren’t the same…

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