כדאי: Worth It, Should | Hebrew Grammar

Modal כדאי says that an action is worth doing or would be a good idea. It gives advice without the necessity of צריך or the obligation of חייב. In this use כדאי is impersonal and invariant: name the person with a dative form such as לי or לך, never by making that person its grammatical subject.

Three shapes of the advice frame

Shape	Use	Model
כדאי + infinitive	general advice; performer left open	כדאי להזמין מראש
כדאי + dative + infinitive	advice directed to a named participant	כדאי לך לנוח
כדאי ש־ + future	advice expressed as a finite clause	כדאי שתזמין מראש

כדאי plus an infinitive

With no named participant, the advice applies generally or to whoever is salient in the conversation. The infinitive names the recommended action.

כדאי להזמין שולחן לפני סוף השבוע.

kedái lehazmín shulkhán lifnéi sof hashavúa

It's worth booking a table before the weekend.

לא כדאי להשאיר את המכונית כאן בלילה.

lo kedái lehash'ír et hamekhonít kan baláyla

It isn't a good idea to leave the car here overnight.

לא כדאי negates the advisability, not the action itself. כדאי לא להתקשר and לא כדאי להתקשר often reach the same practical recommendation, but their structures differ: the first recommends not calling; the second says calling is not advisable. The scope contrast is handled on negating modals.

כדאי plus a dative participant

Add a ל־ pronoun to say who would benefit from following the advice. The modal itself does not agree: כדאי לי, כדאי לך, כדאי לה, כדאי לנו.

כדאי לך לנוח היום, אתה נשמע צרוד.

kedái lekhá lanúakh hayóm, atá nishmá tsarúd

You should rest today; you sound hoarse. (to one man; advice, not an order)

כדאי לנו לצאת מוקדם לפני שהכבישים יתמלאו.

kedái lánu latsét mukdám lifnéi shehakvishím yitmal'ú

We'd be better off leaving early before the roads fill up.

English puts you or we in subject position in these translations. Hebrew treats the person as the beneficiary or affected participant of an impersonal judgment. That is why it says כדאי לך, exactly like מותר לך and קשה לך. The full dative paradigm is on ל־ with pronouns.

💡

Translate כדאי לך as “it would be worthwhile for you.” That English paraphrase preserves the Hebrew structure and keeps you from producing *אתה כדאי.

כדאי ש־ plus a future form

When the performer needs to be an overt subject inside a full clause, use כדאי ש־ followed normally by a future form. The future here is the standard modal complement; it does not necessarily mean that the advice concerns a distant future.

כדאי שתזמין מקום מראש, המסעדה קטנה.

kedái shetazmín makóm merósh, hamis'adá ktaná

You should book ahead; the restaurant is small. (to one man)

כדאי שנבדוק את הכתובת לפני שיוצאים.

kedái shenivdók et haktóvet lifnéi sheyots'ím

We should check the address before we leave.

The ש־ version is often more direct than the infinitive version because it names the person through the finite verb: כדאי להזמין presents an advisable action; כדאי שתזמין addresses a concrete recommendation to “you.” Intonation and context still matter, but the second commonly feels firmer—just short of an instruction.

כדאי שתדברי איתה היום, לפני שהעניין יסתבך.

kedái shedabrí itá hayóm, lifnéi sheha'inyán yistabékh

You should speak to her today, before the matter gets complicated. (to one woman; firm recommendation)

Advice, need, and obligation are not one scale in English

English should is unusually broad. It can express a gentle recommendation, a practical necessity, or a moral obligation. Hebrew makes the force more visible.

Hebrew	Core force	Structure
כדאי	beneficial, worthwhile, advisable	frozen frame; optional dative
צריך	need, practical necessity, ordinary obligation	agrees with a subject
חייב	strong obligation, must	agrees with a subject

כדאי לך לקחת מטרייה, אולי ירד גשם.

kedái lekhá lakákhat mitriyá, ulái yeréd géshem

You should take an umbrella; it might rain. (to one man; prudent advice)

אתה צריך לקחת דרכון, אחרת לא ייתנו לך לעלות למטוס.

atá tsaríkh lakákhat darkón, akhéret lo yitnú lekhá la'alót lamatós

You need to take a passport, otherwise they won't let you board the plane. (to one man; practical necessity)

Translate every English should as צריך and advice becomes obligation. Translate every must as כדאי and a rule becomes a suggestion. צריך and the stronger obligation modals have their own paradigms and complement frames.

Past and future: היה כדאי and יהיה כדאי

Modal כדאי has no tense forms. In the past and future, היה supplies time and stays masculine singular because the frame is impersonal: היה כדאי, יהיה כדאי. The person who might act does not control agreement.

בדיעבד היה כדאי לשאול עוד שאלה לפני שחתמנו.

bedi'avád hayá kedái lish'ól od she'elá lifnéi shekhatámnu

In hindsight, it would have been worth asking one more question before we signed.

אם המחירים ימשיכו לרדת, יהיה כדאי לחכות עוד שבוע.

im hamekhirím yamshíkhu larédet, yihyé kedái lekhakót od shavúa

If prices keep falling, it will be worth waiting another week.

More complex tense and counterfactual readings belong to modals in the past and future.

The hidden pair: כְּדַאי and כְּדָאִי

Unpointed Hebrew writes both words כדאי, but the Academy of the Hebrew Language gives them separate dictionary entries with different parts of speech and different vocalization:

Pointed word	Reading	Category	Behavior
כְּדַאי	kedái	adverbial/modal	invariant: כדאי לי ללכת
כְּדָאִי	keda'í	adjective “worthwhile”	agrees: כדאית, כדאיים, כדאיות

The stress sits at the end in both citation readings, but not on the same vowel: modal ked-ÁI ends in the diphthong ai, whereas adjective keda-Í has a separate final i. The unpointed spelling conceals both the vowel pattern and the syllable boundary.

The Academy's entry for כְּדַאי labels it תואר הפועל and defines the ל־/infinitive and ש־/future frames. Its separate entry for כְּדָאִי labels it שם תואר, defines it as משתלם, and gives the inflected feminine כדאית. The Academy's noun-inflection decision further requires the adjective series כְּדָאִית, כְּדָאִיִּים, כְּדָאִיּוֹת. These are two entries, not one modal agreeing when convenient.

Gender-number	Adjective	Reading
m. sg	כְּדָאִי	keda'í
f. sg	כְּדָאִית	keda'ít
m. pl	כְּדָאִיִּים	keda'iyím
f. pl	כְּדָאִיּוֹת	keda'iyót

זאת עסקה כדאית רק אם אין עמלה נוספת.

zot iská keda'ít rak im ein amlá noséfet

This is a worthwhile deal only if there is no additional fee. (adjective; feminine agreement)

אלה תנאים כדאיים לעסק קטן בתחילת הדרך.

éle tna'ím keda'iyím le'ések katán bitkhilát hadérekh

Those are advantageous terms for a small business starting out. (adjective; masculine plural agreement)

💡

With an action, use invariant modal כְּדַאי: כדאי לבדוק. With a noun being evaluated as profitable or worthwhile, use agreeing adjective כְּדָאִי: השקעה כדאית. In unpointed text, syntax is your only visible clue.

Common Mistakes

1. Giving modal כדאי a subject on the model of English “I should.” Use a dative participant.

❌ אני כדאי לנוח היום

Incorrect — modal כדאי has no subject and never agrees. Say כדאי לי לנוח.

✅ כדאי לי לנוח היום ולחזור לעבודה מחר.

kedái li lanúakh hayóm velakhazór la'avodá makhár

I should rest today and go back to work tomorrow.

2. Using כדאי for a genuine requirement. Advice and necessity are both grammatical, so use a two-✅ contrast and preserve the intended force.

✅ כדאי להגיש את הטופס היום כדי לקבל תשובה מוקדם.

kedái lehagísh et hatófes hayóm kedéi lekabél tshuvá mukdám

It would be wise to submit the form today to get an early answer. (advice)

✅ צריך להגיש את הטופס היום, זה המועד האחרון.

tsaríkh lehagísh et hatófes hayóm, ze hamo'éd ha'akharón

The form has to be submitted today; it's the deadline. (requirement)

3. Conjugating כדאי as though it were a verb. The modal has no past form; the impersonal past frame is היה כדאי.

❌ כדאיתי לשמור את הקבלה

Incorrect — כדאי has no first-person past. Say היה כדאי לי לשמור את הקבלה.

✅ היה כדאי לי לשמור את הקבלה, ביקשו אותה כשהחזרתי את המוצר.

hayá kedái li lishmór et hakabalá, bikshú otá kshehekhzárti et hamutsár

It was worth keeping the receipt; they asked for it when I returned the product.

4. Leaving the adjective uninflected. A noun such as עסקה or השקעה requires feminine כדאית; this is not the frozen modal frame.

❌ ההשקעה הזאת כדאי

Incorrect — השקעה is a feminine noun, so the adjective is כדאית. In pointed text it is כְּדָאִית, not modal כְּדַאי.

✅ ההשקעה הזאת כדאית רק לטווח ארוך.

hahashka'á hazót keda'ít rak litvákh arókh

This investment is worthwhile only in the long term. (adjective)

Key Takeaways

Modal כְּדַאי is invariant and expresses advice or worthwhile action, weaker than צריך or חייב.
Its three productive shapes are כדאי + infinitive, כדאי + dative + infinitive, and כדאי ש־ + future.
The ש־ frame names the advised person in a finite verb and commonly sounds firmer than an impersonal infinitive.
Negate advisability with לא כדאי; watch whether לא scopes over the modal or the action.
Past and future use impersonal היה כדאי and יהיה כדאי.
The Academy separates modal/adverbial כְּדַאיkedái from adjective כְּדָאִיkeda'í, whose forms are כדאית, כדאיים, כדאיות.

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