Learn Hardcore Greek: Δεν πιστεύω ότι θα αργήσει, όμως δεν είμαι σίγουρη. - I don’t believe she will be late, however I am not sure.

ότι introduces a content clause (that-clause): it means “that.” Example: Δεν πιστεύω ότι θα αργήσει = “I don’t believe that he/she/it will be late.”
πως can often replace ότι after verbs of saying/thinking, with little to no change in meaning or formality: Δεν πιστεύω πως θα αργήσει.
ό,τι (with a comma) means “whatever/anything that.” Different word and meaning: Ό,τι πεις = “Whatever you say.” Tip: If you can replace it with “that,” use ότι; if it means “whatever,” it’s ό,τι (with the comma).

Why is it θα αργήσει and not αργεί or θα αργεί?

Greek distinguishes aspect:

θα αργήσει (perfective future) = a single event in the future: “(he/she/it) will be late (on this occasion).”
θα αργεί (imperfective future) = ongoing or habitual future: “(he/she/it) will be running late / will tend to be late.”
αργεί (present) = “(he/she/it) is late (now) / is usually late.” Here the speaker predicts one future occurrence, so perfective future (θα αργήσει) is natural.

What exact form is αργήσει? It looks “subjunctive”—why is it used with θα?

Modern Greek forms the simple future with θα + perfective (subjunctive) stem. So:

Verb: αργώ (to be late)
3rd sg perfective subjunctive: (να) αργήσει
Simple future 3rd sg: θα αργήσει The same perfective form appears after να (e.g., να αργήσει) and after θα for the simple future.

Could I say Δεν πιστεύω να αργήσει? Is it the same?

It’s idiomatic but slightly different in nuance. Δεν πιστεύω να αργήσει often sounds like “I don’t expect/hope (for) him/her to be late,” a bit softer and more about expectation/hope than a straight belief statement. Your original with ότι is a direct belief about a fact.

Why is it δεν and not μην?

δεν negates the indicative (facts/statements), including clauses with θα: Δεν πιστεύω, Δεν είμαι, δεν θα αργήσει.
μην negates the subjunctive/imperative-like environment, typically with να/ας: να μην αργήσει, ας μην αργήσει. Rule of thumb: use δεν with θα and plain present/past; use μην with να/ας.

Is Δεν πιστεύω ότι θα αργήσει the same as Πιστεύω ότι δεν θα αργήσει?

They convey essentially the same belief (“I think he/she won’t be late”), but the emphasis shifts:

Δεν πιστεύω ότι θα αργήσει stresses the lack of belief in the lateness.
Πιστεύω ότι δεν θα αργήσει states a positive belief in non-lateness. Both are natural.

What does όμως add here? Could I use αλλά or μα instead?

όμως = “however/nevertheless,” often set off by commas. It adds a contrastive, slightly formal connector.
αλλά = “but,” the default coordinating conjunction; more neutral when simply opposing two clauses.
μα = colloquial “but,” more informal/emphatic. You could say: …θα αργήσει, αλλά δεν είμαι σίγουρη. With όμως, you get a “however” feel.

Is the comma before όμως correct? Could I make it a new sentence?

Yes. Both are fine:

Single sentence with a comma: …θα αργήσει, όμως δεν είμαι σίγουρη.
Two sentences: …θα αργήσει. Όμως, δεν είμαι σίγουρη. The second version makes the contrast slightly stronger.

Why is it σίγουρη and not σίγουρος?

Adjectives agree with the subject in gender and number. The implied subject of είμαι is the speaker. A female speaker says σίγουρη, a male speaker σίγουρος. Neuter would be σίγουρο (e.g., Δεν είναι σίγουρο = “It’s not certain”).

Who is the subject of θα αργήσει? There’s no pronoun.

Greek typically drops subject pronouns when context makes them clear. θα αργήσει can mean “he/she/it will be late,” depending on prior context. If you need to specify, add a noun or pronoun: Αυτός/Αυτή θα αργήσει, Το λεωφορείο θα αργήσει, etc.

Can I use που instead of ότι/πως after πιστεύω?

Not here. που is mainly a relative pronoun (“who/that/which”) or a colloquial subordinator in certain patterns, but after verbs of thinking/saying, standard Greek uses ότι or πως: Πιστεύω ότι/πως…. Πιστεύω που… is not standard.

Are there good synonyms for parts of this sentence?

Δεν πιστεύω / Δεν νομίζω / Νομίζω πως δεν… (believe/think)
θα καθυστερήσει (synonym of θα αργήσει)
όμως / αλλά / ωστόσο / παρ’ όλα αυτά (however/but)
δεν είμαι βέβαιη/σίγουρη (certain/sure; βέβαιη is a bit more formal) Choose based on tone and register.

Any pronunciation tips for tricky bits?

δεν = then (voiced “th”)
πιστεύω = pee-STÉ-vo (ευ = “ev” before a vowel)
θα = tha (unvoiced “th”)
αργήσει = ar-YÍ-si (γ before η sounds like a soft “y/gh”)
όμως = Ó-mos
σίγουρη = SÍ-ghu-ri (γ = voiced “gh” before ου) Stress the syllables with accents.

Why not say Δεν είμαι σίγουρη ότι… instead of using όμως?

You can. For example: Δεν είμαι σίγουρη ότι θα αργήσει or Πιστεύω ότι δεν θα αργήσει, δεν είμαι όμως σίγουρη. Using όμως creates a clearer contrast between two separate assessments; embedding with ότι makes one longer statement of uncertainty. Both are natural, just different packaging.

Is there any subtle register or politeness effect?

Yes. Greek often softens assertions with hedges:

Δεν πιστεύω ότι… (confident, but not absolute)
όμως δεν είμαι σίγουρη (hedges the claim, polite) You can further soften with μάλλον (probably): Μάλλον δεν θα αργήσει, όμως δεν είμαι σίγουρη.