כש־ is when and אם is if, and that translation is correct as far as it goes. What it hides is that the difference is not about time at all — it is about what the speaker is taking for granted. כש־ says the thing is going to happen and only the timing is in play; אם says the thing itself is an open question. Choosing between them is therefore not a grammar decision but a statement about how sure you are, which is why the same sentence works with either one and means something different each time. Each word has a page of its own — כש־ and כאשר and אם — and the machinery around them is on temporal clauses and real conditionals.
The test
Try whenever in the English. If the sentence survives, Hebrew wants כש־. If you would naturally have said in case or assuming, Hebrew wants אם.
כשאגיע הביתה אתקשר אליך.
kshe'agía habáyta etkashér elékha
When I get home I'll call you. (to a man — I am getting home; the only open question is when)
אם אגיע בזמן, אתקשר אליך מהתחנה.
im agía bazmán, etkashér elékha mehatakhaná
If I get there on time, I'll call you from the station. (to a man — and I may well not)
Note that כש־ is a prefix, written joined to the next word with no space — כשאגיע, כשהוא, כשהייתי — exactly like ש־, which is in fact what it is made of: כ + ש.
The same word covers habits and the past
English uses when for a single future event, for a habit, and for a point in the past, and Hebrew does the same with כש־. Nothing changes in the conjunction; the tense of the verb does the work.
כשהוא בא אנחנו תמיד מזמינים אותו לאכול.
kshehú ba anákhnu tamíd mazminím otó le'ekhól
Whenever he comes over we always end up asking him to stay and eat. (habit — present tense in both halves)
כשהייתי ילד לא היו טלפונים ניידים בכלל.
kshehayíti yéled lo hayú telefoním nayadím bikhlál
When I was a kid there were no mobile phones at all. (a point in the past — past tense in both halves)
כאשר is the same word in a formal coat. It is what you meet in a letter from a clinic, a company announcement or a contract; in speech it sounds stiff. Milog defines it as מילת קישור המציינת זמן, תנאי או התאמה — a linking word marking time, condition or correspondence — so it is not purely temporal even in the dictionary's own account. Do not conclude from that that כש־ and אם are interchangeable: the conditional reading of כאשר belongs to generic and formal statements, not to a plan you are making with a friend.
נעדכן אתכם כאשר יתקבלו התוצאות.
ne'adkén etkhém ka'ashér yitkablú hatotsa'ót
We will update you when the results come in. (formal written — spoken Hebrew says כשיתקבלו)
The real content of this page: Hebrew puts the future inside the clause
Here is the thing to unlearn. English has a rule that a when- or if-clause about the future must nonetheless take a present-tense verb: when I *get home, not *when I will get home; if it rains, not if it will rain. That rule is a quirk of English. Hebrew does not have it. If the event is in the future, the verb inside the clause is in the future, and so is the verb outside it.
| English | Hebrew |
|---|---|
| when I get home — present | כשאגיע הביתה — future |
| if you don't come — present | אם לא תבוא — future |
| when you have finished — perfect | כשתסיימי — future |
So the fix is not to learn a new construction. It is to switch an English constraint off: there is nothing in Hebrew forbidding a future verb after כש־ or אם, and putting one there is the ordinary, unmarked thing to do. Milog's own illustrations do it without comment — אודיע לך כאשר אקבל תשובה, אסלח לו רק אם הוא יתנצל בפני, אם לא תבוא, אלך לבד.
כשתסיימי את הטופס, תשאירי אותו על השולחן.
kshetesaymí et hatófes, tash'íri otó al hashulkhán
When you've finished the form, leave it on the desk. (to a woman — Hebrew future where English has a perfect)
אם לא תבוא עד שמונה, נתחיל בלעדיך.
im lo tavó ad shmóne, natkhíl bil'adékha
If you're not here by eight, we'll start without you. (to a man)
The choice states a belief
This is where the page earns its place, because both of the following are perfectly well-formed Hebrew and they say different things about the speaker.
כשאמות, תמכרו את הדירה ותחלקו ביניכם.
kshe'amút, timkerú et hadirá vetakhlekú beinékhem
When I die, sell the flat and split it between you. (to a group — my death is a fact being planned around)
אם אמות לפני שאסיים את זה, תדאגו שמישהו ימשיך.
im amút lifnéi she'asayém et ze, tid'agú shemíshehu yamshíkh
If I die before I finish this, make sure somebody carries it on. (to a group — now it is a contingency, and it is the finishing that is being planned around)
Nobody thinks the second speaker is unsure whether they will ever die. אם has narrowed the open question to dying before finishing — and that is exactly the work these two words do. Hebrew even has a fixed phrase for refusing to choose: אם וכאשר, "if and when", which Milog lists as an expression in its own right and which Israelis use for a thing that may or may not happen and whose timing is anybody's guess.
Common Mistakes
1. A present tense inside the clause for a single future event. The direct calque from English, and the commonest error on this page. The Hebrew you produce is grammatical — it just says something else.
❌ כשאני מגיע הביתה, אתקשר אליך
when I get home I'll call you
Incorrect as a plan — a present tense in a כש־ clause makes it a habit: 'whenever I get home, I call you'. For one future event Hebrew uses the future in both halves: כשאגיע הביתה אתקשר אליך.
כשאגיע לתל אביב אשלח לך הודעה.
kshe'agía letél avív eshlákh lekhá hoda'á
When I get to Tel Aviv I'll message you. (to a man)
2. Choosing by the English word rather than by what you believe. Both of the following are ✅. The party is a certainty in the first and a gamble in the second, and nothing else in the sentence tells you which.
כשהוא יחזור מהמילואים נעשה מסיבה.
kshehú yakhazór mehamilu'ím na'asé mesibá
When he gets back from reserve duty we'll throw a party. (he is coming back; only the date is open)
אם הוא יחזור מהמילואים בזמן, נעשה מסיבה.
im hu yakhazór mehamilu'ím bazmán, na'asé mesibá
If he gets back from reserve duty on time, we'll throw a party. (now the party itself is in doubt)
3. Writing כש־ as a separate word. It is a prefix, and a prefix cannot stand on its own.
❌ כש אני עייף אני לא שווה כלום
Incorrect as a spelling — כש־ is written joined to the following word: כשאני. It is built from כ + ש־, and ש־ is itself a prefix.
כשאני עייף אני לא שווה כלום.
kshe'aní ayéf aní lo shavé klum
When I'm tired I'm useless. (m. speaker)
4. Using כש־ for whether. English if does two jobs — the conditional and the indirect question — and only one of them is כש־'s territory. Neither of them, in fact: both belong to אם.
❌ תשאל אותו כשהוא בא
ask him whether he's coming
Incorrect as a reading — כש־ is never 'whether'. As written this says 'ask him when he comes', i.e. do the asking at that moment. Indirect questions take אם: תשאל אותו אם הוא בא.
תשאל אותו אם הוא בא, אני כבר לא מצליח לתפוס אותו.
tish'ál otó im hu ba, aní kvar lo matslíakh litpós otó
Ask him whether he's coming — I can't get hold of him any more. (to a man, m. speaker)
Key Takeaways
- The split is presupposition, not time: כש־ takes the event for granted and leaves only the timing open; אם leaves the event itself open.
- The test that works in English: whenever → כש־, in case → אם.
- כש־ is a prefix. כשאני, כשהוא, כשתסיימי — never a space. כאשר is its formal written twin.
- Hebrew puts the future inside the clause. English forbids when I will get home; Hebrew requires כשאגיע. This is an English constraint to switch off, not a Hebrew form to learn.
- The rule is about reference: a clause about now keeps the present (אם אתה רוצה), a clause about later takes the future.
- The same sentence is well-formed with either word and states a different belief — כשאמות against אם אמות. Nothing but your choice of conjunction carries it.
- אם also means whether, for indirect questions. כש־ never does.
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Start learning Hebrew →Related Topics
- כש־ and כאשר: WhenA2 — כש־ is the everyday when — an attached prefix taking a whole clause, as in כשהגעתי הביתה גיליתי שאין חשמל — and כאשר is its formal twin with identical syntax. The rule that decides whether your sentence works is the tense: for a future event Hebrew puts a future verb inside the clause, כשנחזור מהחופשה נתחיל לחפש דירה, exactly where English forces a present.
- אם: If and WhetherA2 — אם does two unrelated jobs with the same three letters: it opens a real condition, אם תבוא בשמונה נספיק לאכול, and it embeds a yes-no question, שאלתי אותו אם הוא מכיר מישהו. English merges the same two jobs into if, which is why the split is invisible rather than difficult — and why האם, which looks like a stronger אם, does neither of them the same way.
- Real Conditionals with אםA2 — An open condition opens with אם, and the one thing you have to get right is the tense: Hebrew puts a FUTURE verb in both halves — אם ירד גשם, נישאר בבית — exactly where English bans its own future. Habitual conditions take the present in both halves and past-fact conditions take the past, and the optional אז that opens the main clause is spoken Hebrew.
- Temporal Clauses and Their TenseB1 — After כש־, אחרי ש־, לפני ש־, עד ש־, מאז ש־ and בזמן ש־, Hebrew puts a future verb in the clause when the event is in the future — אחרי שאסיים, אלך הביתה — where English forces a present. All six need a ש in front of a clause, and their bare twins לפני, אחרי and בזמן take a noun instead.
- לפני ש־, אחרי ש־, עד ש־, מאז ש־A2 — Hebrew builds its time connectives by bolting ש onto a preposition: לפני הארוחה takes a noun, לפני שאכלנו takes a whole clause, and the ש is the only thing that tells them apart. This page covers the six of them — לפני ש, אחרי ש, עד ש, מאז ש, ברגע ש and כל פעם ש — plus the written בטרם and לאחר ש, and the exclamative עד שהגעת!
- Using the Future: Prediction, Intention, RequestA2 — One prefix conjugation covers everything English splits between will, shall, going to and the polite question — and it supplies the everyday command on top. Nothing in the form separates those readings, so this page is about what does.