כש־ and כאשר: When | Hebrew Grammar

כש־ is the word you will use every day for when, and כאשר is the same word in a suit. Both take a full clause, both work in every tense, and neither adds anything the other lacks. What does need learning is the tense inside the clause — and that is a matter of switching an English rule off rather than learning a Hebrew one. The wider set of temporal connectives is on temporal conjunctions, the tense system across all of them on temporal clauses, and the when-versus-if decision on כש־ or אם?.

A prefix that swallows whatever comes next

כש־ is written joined, with no space, and it attaches straight over the following word's own prefix — כשה־, כשב־, כשי־.

כשהגעתי הביתה גיליתי שאין חשמל.

kshehigáti habáyta gilíti she'éin khashmál

When I got home I found there was no electricity.

כשהילד נרדם סוף סוף, ישבנו לאכול.

kshehayéled nirdám sof sof, yashávnu le'ekhól

When the little one finally fell asleep, we sat down to eat.

כשבחוץ קר אנחנו מדליקים את התנור בסלון.

kshebakhúts kar anákhnu madlikím et hatanúr basalón

When it's cold outside we put the heater on in the living room.

That behaviour is not special to כש־ — it belongs to the ש־ family, of which כש־ is a member.

Neither word comes from the other

The story you will be told is that כש־ is a shortened כאשר. It is not. They are parallel formations built on the same preposition: כְּשֶׁ־ is כְּ־ plus the relative שֶׁ־, and כַּאֲשֶׁר is כְּ־ plus the relative אֲשֶׁר. Hebrew has two relative markers, and each of them takes the same preposition to make a when.

And כש־ is not a modern shortcut either. It is sitting in the Mishnah, in one of the most quoted lines in the language.

אם אין אני לי, מי לי? וכשאני לעצמי, מה אני?

im ein aní li, mi li? ukhshe'aní le'atsmí, ma aní?

If I am not for myself, who is for me? And when I am for myself, what am I? (Pirkei Avot 1:14 — כש־ in a text redacted around 200 CE)

The Academy of the Hebrew Language sets them side by side rather than deriving one from the other. In its ruling כְּשֶׁ־ ולא שֶׁ־ (read from a Wayback snapshot of 10 September 2025): הרצף כְּשֶׁ־, בדיוק כמו כַּאֲשֶׁר, פותח פסוקית זמן ועונה על השאלה 'מתי' — the sequence כְּשֶׁ־, exactly like כַּאֲשֶׁר, opens a time clause and answers the question when. What separates them is register, nothing else.

The tense rule, which is the whole difficulty

For a future event, the verb inside the כש־ clause goes into the future, and so does the verb outside it. English will not let you do this — when I get home, never when I will get home — and that prohibition is a quirk of English syntax rather than a fact about time clauses. No authority states the Hebrew side as a codified rule, and it does not need one: this is simply what people say.

כשנחזור מהחופשה נתחיל לחפש דירה.

kshenakhazór mehakhufshá natkhíl lekhapés dirá

When we get back from holiday we'll start looking for a flat.

כשנגיע למלון נשאיר את המזוודות ונצא ישר לים.

kshenagía lamalón nash'ír et hamizvadót venetsé yashár layám

When we get to the hotel we'll drop the suitcases and head straight for the beach.

כשיהיה לכם תאריך סופי, תגידו לנו ונזמין טיסות.

ksheyihyé lakhém ta'aríkh sofí, tagídu lánu venazmín tisót

When you've got a firm date, tell us and we'll book flights. (to a group)

Past and habitual: the conjunction does not move

Nothing about כש־ changes across the tenses. A finished event takes the past, and a standing generalisation takes the present — the same instruction applied to different facts. Here is a stretch of past narrative, three sentences of it:

כשיצאנו מהבית עוד היה אור.

ksheyatsánu mehabáyit od hayá or

When we set out it was still light.

כשהגענו לתחנה הרכבת כבר עזבה.

kshehigánu latakhaná harakévet kvar azvá

By the time we got to the station the train had already gone.

וכשסוף סוף עלינו על הבאה, גילינו שהיא בכלל לא עוצרת אצלנו.

ukhshesóf sof alínu al haba'á, gilínu shehí bikhlál lo otséret etslénu

And when we finally got on the next one, we found it didn't stop at our station at all.

כשיש שרב אני לא יוצא מהבית עד הערב.

ksheyésh sharáv aní lo yotsé mehabáyit ad ha'érev

When there's a heatwave I don't leave the house till the evening. (m. speaker — a standing habit, so the present)

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Hebrew does not split when from while the way English does — כש־ covers both, and the past narrative above would use it for a point in time and for a stretch of one alike. When genuine simultaneity has to be marked, Hebrew reaches for בזמן ש־, which is on durative conjunctions.

A כש־ clause often has no verb in it

Hebrew has no present-tense copula, so the clause after כש־ regularly consists of a subject and a predicate with nothing conjugated between them. It looks incomplete to an English eye and is entirely regular.

כשהכול סגור בשבת אני מבשל בבית.

kshehakól sagúr beshabát aní mevashél babáyit

When everything's shut on Saturday I cook at home. (m. speaker — no verb at all inside the כש־ clause)

Note also what כש־ is not. It never translates the when of a question: when are you coming? is מתי אתה מגיע, with a question word, and the same מתי stays put when the question is embedded — on indirect questions.

כאשר, and how far apart the two really are

כאשר is a free word with identical syntax, and it belongs to written and formal Hebrew: a clinic letter, a company notice, a contract, a printed instruction.

כאשר יסתיים הבירור תישלח אליכם הודעה בדואר.

ka'ashér yistayém haberúr tishalákh aleikhém hoda'á badó'ar

When the inquiry is concluded a notice will be sent to you by post. (formal written)

כשייגמר הבירור ישלחו לכם מכתב הביתה.

ksheyigamér haberúr yishlekhú lakhém mikhtáv habáyta

When the inquiry's over they'll send you a letter. (the same message, ordinary Hebrew)

The size of the gap is measurable. On Hebrew Wikipedia, with the regex form of insource: and one space on each side of every probe so that both columns are caught by the same net (17 August 2026):

Frame	כאשר	כש־
third-person past — הגיע higía	1,076–1,102	1,207
first-person present — אני aní	85–90	486

The contrast between the two rows is the finding. In third-person past narration — the encyclopedia's own voice — the two are level. In the first-person present, which is where quoted speech lives, כש־ leads better than five to one. The four-digit figures are floors rather than counts, but the gap between the rows survives that.

The one thing the Academy rules against

You will hear the kaf dropped constantly: שהגעתי הביתה for כשהגעתי הביתה. The Academy's position is explicit — בפתיחה של תיאור זמן נכון לומר 'כְּשֶׁ־', at the head of a time expression the correct form is כְּשֶׁ־, without dropping the kaf — and it sets against it the plain ש־, which opens clauses answering איזה or מה rather than מתי. It notes that the reduced form is widespread in speech and is working its way into writing, and diagnoses it as phonetic: the k is squeezed out of an unvocalized כש cluster.

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The Academy reports the scholarly argument rather than burying it, and so should this page. Moshe Bar-Asher found a handful of ש-for-כש readings in Mishnah manuscripts and read them as a rare early time-clause ש־; Menachem Zvi Kaddari replied that those clauses were originally relative or causal and were later "corrected" by adding the kaf. The dispute is about the Mishnah. For your own writing the instruction is not in dispute: write כש־.

Common Mistakes

1. A present tense in the clause for one specific future event. The direct English calque, and the error this page exists for. What you produce is well-formed Hebrew — it has just turned your plan into a routine.

❌ כשהיא מסיימת את הקורס, היא מחפשת עבודה

when she finishes the course she'll look for a job

Incorrect as a plan — the present in both halves reads as a routine, 'whenever she finishes a course she looks for a job'. For this one course both verbs go into the future: כשהיא תסיים את הקורס היא תחפש עבודה.

✅ כשהיא תסיים את הקורס היא תחפש עבודה בתחום.

kshehí tesayém et hakúrs hi tekhapés avodá batkhúm

When she finishes the course she'll look for a job in the field.

2. Putting כש־ after a time noun. English the day when I arrived has a when in it, so the hand supplies one. Hebrew does not: a clause hanging on a noun is a relative clause and takes a plain ש־.

❌ ביום כשהגעתי לתל אביב לא הכרתי אף אחד

on the day I arrived in Tel Aviv I didn't know a soul

Incorrect as that sentence — a time noun takes a plain relative ש־: ביום שהגעתי לתל אביב. As written, ביום can only be read 'in the daytime', which is not what the sentence is about.

✅ ביום שהגעתי לתל אביב לא הכרתי שם אף אחד.

bayóm shehigáti letél avív lo hikárti sham af ekhád

On the day I got to Tel Aviv I didn't know a single person there.

3. Adding a ש to כאשר. A reasonable over-generalisation — אחרי ש־, לפני ש־ and עד ש־ all carry one — but כאשר already contains its own.

❌ כאשר שנגיע לתחנה נתקשר אליך

Incorrect — כאשר takes its clause directly and never a second ש: כאשר נגיע לתחנה, or in speech כשנגיע לתחנה.

✅ כאשר נגיע לתחנה נעדכן אתכם בהודעה.

ka'ashér nagía latakhaná ne'adkén etkhém behoda'á

When we get to the station we'll update you by message. (formal)

4. Choosing כאשר because it looks like the proper word. Both of the following are ✅. The first is what you say to a friend; the second is what a printed instruction says, and saying it aloud to someone you know is comic.

תתקשרי אליי כשתגיעי, לא משנה באיזו שעה.

titkashrí elái kshetagí'i, lo meshané be'éizo sha'á

Call me when you get in, whatever time it is. (to a woman)

נא ליצור קשר כאשר תגיעו ליעד.

na litsór késher ka'ashér tagí'u layá'ad

Please make contact when you reach your destination. (formal written — a printed instruction, not a spoken sentence)

Key Takeaways

כש־ is a prefix, written joined and stacking over the next word's own prefix: כשהילד, כשבחוץ.
Neither word derives from the other. כְּשֶׁ־ is כְּ־ plus שֶׁ־ and כַּאֲשֶׁר is כְּ־ plus אֲשֶׁר — the same preposition on Hebrew's two relative markers — and כש־ is already in Pirkei Avot 1:14.
Future reference puts a future verb inside the clause: כשנחזור מהחופשה נתחיל לחפש דירה. This is an English rule to switch off, not a Hebrew one to learn.
A finished event keeps the past (כשהגענו לתחנה) and a habit keeps the present (כשיש שרב אני לא יוצא) — the conjunction never changes.
כאשר is register, not meaning. Measured with one probe shape: level with כש־ in third-person past narration, outnumbered better than five to one in a first-person present clause.
The Academy rules that a time clause opens with כְּשֶׁ־ and not a bare שֶׁ־, noting the dropped kaf is widespread in speech.
A clause on a time noun takes a plain ש־: ביום שהגעתי, never ביום כשהגעתי.

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