Learn Hardcore Japanese: murina yakusoku ha sinaide kudasai. - Please don’t make impossible promises.

無理な means unreasonable, unrealistic, or impossible to carry out. With 約束, it implies promises you can’t realistically keep. It’s not about physical force; it’s about feasibility and reasonableness. Related:

無理をする: to overdo/overexert yourself
無理のない: reasonable, not overtaxing (e.g., 無理のない計画)

Why is it 無理な約束, not 無理の約束?

Because 無理 is a na-adjective when directly modifying a noun, so it takes な: 無理な約束. The pattern 無理の exists in set expressions like 無理のない or 無理のある, but 無理の約束 is not natural.

Why is は used after 約束?

は marks the topic and often adds a contrastive nuance. 無理な約束はしないでください focuses on the category “unreasonable promises,” effectively “As for unreasonable promises, please don’t make them,” implicitly contrasting them with other promises.

Could I use を instead of は (i.e., 無理な約束をしないでください)?

Yes. 無理な約束をしないでください is perfectly natural and treats 約束 as the direct object. Nuance:

は: topical/contrastive emphasis on “unreasonable promises”
を: straightforward “don’t make unreasonable promises” (no special contrast implied)

Why is 約束 used with する? Is 約束する or 約束をする better?

約束 is a する-noun. Both are fine:

約束する (more concise, very common)
約束をする (a bit more deliberate/formal-sounding) In negatives: 約束しないでください and 約束をしないでください are both acceptable.

How does しないでください work grammatically?

It’s the negative request form: ないで + ください = “please do not do.” For する:

Dictionary: する
Negative: しない
Request not to do: しないでください

Can I say しなくてください instead?

No. For a negative request you must use ないでください, not なくてください. The なくて form is used for reasons or for connecting clauses (e.g., “because [someone] didn’t do X”), not for requests.

How strong or polite is しないでください compared to other options?

From strongest/most blunt to softest:

Command: するな (harsh; don’t do it!)
Casual request: しないで / しないでね (softened with ね)
Polite request: しないでください (standard polite)
Softer/more indirect: しないでくれませんか / しないでいただけますか / しないようにしてください Note: しないでくださいませんか is grammatically possible but sounds stiff/old-fashioned; prefer いただけませんか.

How can I soften this further?

Add ね: 無理な約束はしないでくださいね (friendly/soft)
Use ように: 無理な約束はしないようにしてください (please make sure not to…)
Frame as a favor: 無理な約束はなさらないよう、お願いいたします (very polite/formal)

What’s a formal sign/poster alternative?

無理な約束はご遠慮ください。
無理な約束はお控えください。
無理な約束はなさらないようお願いいたします。 These avoid directly saying “you” and sound appropriate in public notices or business contexts.

How is 無理な約束はしないでください different from 無理をしないでください?

無理な約束はしないでください: Don’t make unreasonable/unrealistic promises.
無理をしないでください: Don’t overdo it / Don’t push yourself. Different target; one is about the content of promises, the other about personal exertion.

How do I say “to me” or “with me” in this sentence?

To me (promise to someone): 私に無理な約束はしないでください。
With me (make a promise with someone): 私と無理な約束はしないでください。 Both are used; に emphasizes the recipient, と emphasizes the counterpart.

Does しないで also mean “without doing”? Could that cause confusion?

Yes, ないで can mean “without doing” (e.g., 約束しないで帰った = “went home without making a promise”). When followed by ください, it’s clearly a polite negative request (“please don’t do X”), so there’s no ambiguity here.

Why are there spaces between the words?

They’re for learners’ readability. Standard Japanese writing usually doesn’t use spaces: 無理な約束はしないでください。

Should ください be written in kanji (下さい)?

When used as a polite request after the て-form (or ないで), the conventional style is hiragana: ください. The kanji 下さい is often reserved for the literal “please give me [thing]” (e.g., 水を下さい), though many modern texts still prefer hiragana in both cases.