musn't → mustn't

No. Forms like "musn_t", "musnt", or "musn't" are incorrect. The correct contraction of "must not" is mustn't (must + n't). In formal writing, write must not.

Quick answer

Use mustn't (m-u-s-t-'n-t) or write must not in formal contexts.

"mustn't" = contraction of "must not"; the apostrophe replaces the o.
Common incorrect forms: "musnt", "musn_t", "must'nt", "must n't".
If unsure, use must not - it avoids punctuation mistakes and reads clearly.

Core explanation: what the contraction looks like

Contractions drop letters and mark the gap with an apostrophe. For must + not the correct result is must + n't → mustn't: keep "must", add an apostrophe, then "nt".

Any other symbol, extra space, or misplaced apostrophe makes the token wrong.

Correct: mustn't → m u s t ' n t
Wrong to watch for: musnt, musn_t, must'nt, must n't
Example: Wrong: You musn_t enter the lab without goggles. -
Right: You mustn't enter the lab without goggles.

Hyphenation, apostrophes, and spacing (why musn_t looks wrong)

Apostrophes mark omitted letters. Hyphens or underscores are different characters and break contractions. Contractions must be a single token with an apostrophe: mustn't.

Spaces inside contractions are incorrect and can confuse grammar checkers and readers.

Use the apostrophe character ('), not - or _.
Do not insert spaces: mustn't (correct) vs must n't or must 'nt (wrong).
Examples: Wrong: We must-nt accept that. -
Right: We mustn't accept that.

Contraction grammar: when to use mustn't and when to use must not

Both forms are grammatically correct. Choose by register and emphasis: use mustn't in conversation or informal writing; use must not in formal, legal, or emphatic contexts.

Formal/emphatic: "Employees must not share passwords."
Informal/conversational: "You mustn't miss this!"
Emphasis: "You must not ignore this safety warning."

Formal: Policy: Visitors must not enter restricted areas without an escort.
Informal: Friend chat: You mustn't forget the tickets!

Real usage: where you'll see mustn't and where to avoid contractions

Expect mustn't in dialogue, casual messages, and informal instructions. Avoid contractions in reports, contracts, policies, and academic texts unless the style guide allows them.

Dialogue, text messages, quick emails → mustn't
Reports, contracts, official policies → must not
Examples: Work: Chat: You mustn't push server changes without testing.
School: Syllabus: Students must not submit late assignments without permission.
Casual: Text: You mustn't be late to brunch!

Examples across contexts - wrong/right pairs you can copy

Below are typical mistyped forms and direct corrections. Copy the correct line into your draft or swap the contraction for must not in formal text.

Work wrong: Musnt we include the budget figures in the summary?
Work right: Mustn't we include the budget figures in the summary?
Work formal: Shouldn't we include the budget figures in the summary? / We must not omit the budget figures.
Work wrong: I musnt attach the file until Ben signs off.
Work right: I mustn't attach the file until Ben signs off.
Work alt: I must not attach the file until Ben signs off.
School wrong: Students musnt use calculators during the test.
School right: Students mustn't use calculators during the test.
School alt: Notice: Students must not use calculators during the test.
School wrong: You musn_t leave your seat before the bell.
School right: You mustn't leave your seat before the bell.
Casual wrong: Musn't forget to bring snacks.
Casual right: Mustn't forget to bring snacks.
Casual alt: Don't forget to bring snacks.
Casual wrong: He musnt know about the surprise.
Casual right: He mustn't know about the surprise.
Meta wrong: They musn't've seen the update.
Meta right: They mustn't have seen the update.

Try your own sentence

Check the whole sentence, not just the phrase. Context usually clarifies whether a contraction fits your tone.

Rewrite help: three copy-ready patterns to fix your sentence

Pick a pattern based on tone and clarity: quick contraction fix, formal expansion, or a rephrase.

Pattern 1 - Quick fix: Replace musnt / musn_t / must'nt → mustn't.
Pattern 2 - Formal expansion: Replace contraction with must not.
Pattern 3 - Rephrase for clarity: Use Don't + verb or a direct instruction.

Rewrite 1: Wrong: We musnt ignore customer comments. → Quick fix: We mustn't ignore customer comments.
Rewrite 2: Wrong: Students musnt submit late work. →
Formal: Students must not submit late work.
Rewrite 3: Wrong: You musn_t miss this meeting. → Rephrase: Don't miss this meeting.

Memory tricks and quick checks

Mnemonic: "the apostrophe stands where the O went" - must not → mustn't (apostrophe before n).

Fallback: If punctuation worries you, write must not. It removes the contraction decision.

If you typed musnt, expand to must not, then contract carefully to mustn't.
On mobile, type the apostrophe manually if autocorrect removes it.
Tip: Press the apostrophe key instead of hyphen/underscore when fixing contractions.

Similar mistakes to watch for

The same pattern applies to other contractions: either the apostrophe is missing or placed incorrectly. Fix by inserting the apostrophe where the omitted letter(s) belong or by expanding the phrase.

cant → can't; wont → won't; didnt → didn't; isnt → isn't
Watch for misplaced apostrophes: must'nt, musn't (apostrophe after s) - both wrong.
Examples: Wrong: I cant finish the report. -
Right: I can't finish the report.
Wrong: They must'nt be late. -
Right: They mustn't be late.

Practical checklist: fix every musn_t in under a minute

Search, replace, and read aloud. A quick pass catches most variants and makes your text consistent.

1) Search for likely strings: musn, musnt, must'nt, musn_t, must n't.
2) Replace each with mustn't or must not depending on tone.
3) Read the sentence out loud; if it sounds awkward, rephrase with Don't or expand to must not.

Work: Edit: "We musnt publish this" → "We mustn't publish this" or "We must not publish this" for the report.
School: Edit: "You musn't cheat on exams" → "You mustn't cheat on exams" or "You must not cheat on exams" on the syllabus.
Casual: Edit: "musn_t forget!" → "mustn't forget!" or "Don't forget!" in a text.

FAQ

Is "musn't" correct?

No. The apostrophe must replace the o in not, producing mustn't. Forms like musn't (apostrophe after s) or musn_t are incorrect.

When should I use mustn't versus must not?

Use mustn't in informal contexts (dialogue, chat). Use must not in formal, legal, or highly emphatic contexts. When unsure, choose must not.

Why do people type musnt or musn_t?

Fast typing, mobile keyboards, autocorrect, or confusing hyphens and apostrophes cause these errors. Formatting conversions can also strip punctuation.

How do I fix many occurrences at once?

Search your document for likely error strings (musn, musnt, must'nt, musn_t) and replace with mustn't or must not. A grammar checker can catch more variants.

Are other contractions affected the same way?

Yes. Common similar mistakes include cant → can't, didnt → didn't, and misplaced apostrophes like must'nt. Apply the same fix: insert the apostrophe where letters were omitted or expand the phrase.

One last quick check before you send

If you're unsure, paste a few sentences into a grammar tool to flag missing apostrophes and odd tokens like musn_t. Fix those, then confirm tone: mustn't for informal, must not for formal, or rephrase with Don't.