comma between 'then' and 'when'

Most of the time, don't put a comma between then and when: write "then when" as a single timing phrase. Use "then, when" only when "then" is an aside or you want a deliberate pause.

Below: short rules, clear tests, many rewrites, and grouped examples for work, school, and casual writing so you can fix sentences fast.

Quick answer

Usually no - write "then when" without a comma. Use "then, when" only if "then" is parenthetical or you want an emphatic pause that changes meaning.

If then + when form one adverbial phrase modifying the verb, do not separate them with a comma.
Set off then with commas only when it interrupts as an aside: e.g., "Then, when I checked, ..."
Quick test: read it aloud. Natural pause → comma; forced pause → remove comma.

Core explanation: why "then when" usually stays together

Then and when often act as a single timing phrase that modifies the main verb. A comma between them usually creates an unnecessary pause or implies a parenthetical comment.

No comma (adverbial phrase): then when I arrive we will start.
Comma (parenthetical then): then, when I arrive, I'll tell you.
Default for neutral prose: omit the comma unless the pause changes the meaning or tone.

Real usage: when a comma before "when" is correct

Use a comma when then interrupts the sentence as an aside or when you need dramatic emphasis. Also use commas to set off truly nonessential clauses.

Parenthetical: Then, when I checked, the number had changed.
Dramatic emphasis: Then, when the lights went out, everyone screamed.
If two independent clauses are involved, prefer a period or semicolon rather than forcing commas.

Spacing and punctuation: read for rhythm, not a formula

Punctuation sets rhythm. Read sentences aloud to judge pauses. If the pause feels forced, remove the comma. Standard spacing: comma followed by a single space; never add extra spaces to fake a pause.

Natural pause = comma; no pause = no comma.
Use dashes or split sentences if you need a stronger break than a comma.

Hyphenation and dashes: when a dash is the better choice

Hyphens join words and don't apply to "then when." Use an em dash sparingly for abrupt interruptions or strong emphasis; it changes tone more than a comma.

Hyphen = not applicable here.
Em dash = stronger interruption: Then-when she finally looked up-everyone laughed.
When clarity is the goal, consider splitting into two sentences rather than overpunctuating.

Grammar section: short structural test

If when begins a clause that tells when the main verb happens and then only signals sequence, they form one unit - no comma. If then interrupts as an aside, set it off with commas.

Test: remove then. If when still connects naturally to the verb, then was likely parenthetical.
If removing then breaks the timing relationship, keep then when together (no comma).
When in doubt for formal writing, omit the comma.

Try your own sentence

Test the whole sentence in context rather than the phrase alone: context usually makes the right punctuation clear.

Fix your sentence: rewrite templates and concrete rewrites

Choose one of these quick fixes: remove the comma, restructure the clause, or split into two sentences. Below are ready-to-use rewrites for email, essays, and texts.

Template A (remove comma): ... then when ...
Template B (restructure): When X happens, Y. / After X, Y.
Template C (split): Do X. Then, when Y happens, do Z.

Rewrite:
Wrong: I will go to the store, then, when I get home, I'll cook dinner. Better: I will go to the store; then when I get home I'll cook dinner.
Rewrite:
Wrong: She checked the code, then, when she ran it, errors appeared. Better: She checked the code, and when she ran it errors appeared.
Rewrite:
Wrong: We'll meet, then, when you're ready, we'll start the review. Better: We'll meet when you're ready to start the review.
Work rewrite: Wrong: Send the file, then, when I approve it, update the tracker. Better: Send the file; when I approve it update the tracker.
School rewrite: Wrong: I ran the test, then, when the solution changed color, I recorded results. Better: I ran the test and recorded results when the solution changed color.
Casual rewrite: Wrong: I'll call you, then, when I'm free, we'll chat. Better: I'll call you when I'm free and we can chat.

Examples: wrong vs right - work, school, and casual

Each wrong sentence has an unnecessary comma between then and when. The right sentence shows a clearer alternative or a deliberate parenthetical.

Work - Wrong: I'll draft the agenda, then, when the team arrives, I'll present the plan.
Right: I'll draft the agenda, then when the team arrives I'll present the plan.
Work - Wrong: We'll approve the budget, then, when accounting signs off, funds will be released.
Right: We'll approve the budget, then when accounting signs off funds will be released.
Work - Wrong: Send the draft, then, when I review it, add comments inline.
Right: Send the draft, then when I review it add comments inline.
School - Wrong: I read the chapter, then, when I finished, I wrote a summary.
Right: I read the chapter, then when I finished I wrote a summary.
School - Wrong: The experiment ran an hour, then, when we tested pH, values shifted.
Right: The experiment ran an hour, then when we tested pH values shifted.
School - Wrong: He'll finish the assignment, then, when the teacher collects it, he'll hand in a paper copy.
Right: He'll finish the assignment, then when the teacher collects it he'll hand in a paper copy.
Casual - Wrong: I'll call you later, then, when I'm on the train, I'll text details.
Right: I'll call you later, then when I'm on the train I'll text details.
Casual - Wrong: Let's eat, then, when we're done, see a movie.
Right: Let's eat, then when we're done see a movie.
Casual - Wrong: I'll grab coffee, then, when you arrive, we'll catch up.
Right: I'll grab coffee, then when you arrive we'll catch up.
Intentional aside (acceptable): Then, when we finally agreed, the meeting ended.

Memory trick: the quick two-step test

Two-step test: (1) Remove then - does the sentence still make sense with when? (2) Read it aloud - do you naturally pause between then and when?

If step 1 = yes and you don't need the pause → drop the comma.
If step 2 = natural pause and removing then breaks meaning → keep the comma.
Default for professional writing: prefer no comma.

Similar mistakes to watch for

Writers who add unnecessary commas between then and when often make nearby errors: comma splices, unnecessary commas after adverbs, or confusing then with than. Fixing then/when usage often helps spot these issues.

Comma splice - don't use a comma alone to join two independent clauses.
Unnecessary commas after adverbs: avoid "however, when" unless the pause is needed.
Then vs than - check whether you need sequence (then) or comparison (than).
Example fix: Wrong: I finished the draft, then, when I emailed it, she replied.
Right: I finished the draft; then when I emailed it she replied.

FAQ

Do you put a comma between then and when?

Usually no. Use "then when" when they form one adverbial phrase. Use "then, when" only if "then" is parenthetical or for deliberate emphasis.

Is it always wrong to write "then, when"?

No. It's fine when "then" is an aside or for narrative emphasis. In formal or neutral prose, avoid the comma unless it affects meaning.

How can I quickly fix sentences that use "then, when" incorrectly?

Run the two-step test: remove then and see if when still links to the verb, then read aloud. If there's no natural pause, remove the comma or restructure the sentence.

Should I use a dash instead of a comma?

Use an em dash for an abrupt interruption or strong emphasis. For clarity in formal writing, prefer no comma or split into two sentences.

Will grammar checkers catch this error?

Many tools flag unnecessary commas, but they miss stylistic choices. Use the two-step test to confirm any automated suggestion.

Want a fast check?

If you're unsure, paste one sentence into a checker or read it aloud. Use the removal test: if when still links naturally, drop the comma. Practice with the example pairs to build an instinct for meaningful pauses versus noise.