ještě ne / už ne

In Czech, temporal polarity operates on a logical 2x2 grid based on state boundaries. The true logical negation of už (already/now) is ještě ne (not yet), and the logical negation of ještě (still) is už ne (no longer). Treating these as 1-to-1 translations of English words breaks down in negative contexts.

• Už (already/now) & Ještě ne- (not yet): Už typically pairs with perfective verbs to show a completed result (already). Crucially, it also frequently pairs with imperfective verbs to mean 'now' (indicating a new state has begun, e.g., Už spí. or Už chápu.), where English speakers might incorrectly use teď. Note that while už with an imperfective verb means 'now' in the present tense, in the past tense it translates back to 'already' (e.g., Už spal, když jsem přišel. - He was already sleeping when I arrived.). Its opposite, ještě ne-, indicates that an awaited result has not been reached, or a state hasn't started yet.
• Ještě (still) & Už ne- (no longer / not anymore): Ještě typically pairs with imperfective verbs to show an ongoing state. Its opposite, už ne-, indicates that a situation or state that previously existed has now ended.

The Q&A Flip:
Because of these cross-diagonal opposites, answering yes/no questions requires a 'flip'. A question with už is answered negatively with ještě ne (e.g., Q: Už jsi jedl? A: Ne, ještě ne.). Conversely, a question with ještě is answered negatively with už ne (e.g., Q: Ještě prší? A: Ne, už ne.).

Morphology and Syntax:
While ještě and už are free-standing adverbs, the negative particle ne- is a bound prefix attached directly to the verb (e.g., už nepracuju). In compound past tenses, it attaches to the l-participle (e.g., ještě jsem neviděl). In the future tense, the prefix attaches to the auxiliary verb for imperfective verbs (e.g., už nebudu pracovat), but directly to the conjugated verb for perfective verbs and determinate verbs of motion (e.g., už to neudělám, už tam nepůjdu). Watch out for the irregular 3rd-person form of být, which fuses into není (e.g., ještě není). Additionally, when ještě or už stand alone as the first syntactic phrase in a sentence, past tense auxiliary clitics must immediately follow them due to the second-position clitics rule (e.g., Už jsem to nedělal.). However, if they modify another word to form a single first-position phrase, the clitic follows the entire phrase (e.g., Už včera jsem to věděl.). These adverbs do not have to start the sentence and frequently sit right before the verb phrase (e.g., Já jsem to ještě neudělal.). If the verb is reflexive, the reflexive pronoun (se/si) must immediately follow the auxiliary as part of the clitic cluster (e.g., Ještě jsem se nerozhodl.). When used as standalone short answers, they are written separately (Ještě ne. / Už ne.).

Nuance: Pořád and Pořád ne
Learners often confuse ještě ne with pořád ne (still not). While ještě ne neutrally states an event hasn't happened yet, pořád ne emphasizes a prolonged duration, which can either neutrally highlight the length of time or express frustration and impatience (e.g., 'He still hasn't arrived!'). Similarly, in positive sentences, pořád or ještě pořád are heavily used in spoken Czech alongside ještě to emphasize an ongoing state (e.g., Ještě pořád prší. - It is still raining.).

Non-Temporal 'Ještě':
Outside of this temporal grid, ještě is also frequently used in quantitative contexts to mean 'more' or 'another' (e.g., ještě jedno pivo - one more beer; Co ještě? - What else?).