Learn Hardcore Greek: Ο καιρός είναι καλός σήμερα.

Greek uses the definite article much more than English, especially with general nouns used as subjects. Talking about today’s weather naturally takes the article: Ο καιρός…. Leaving it out here would sound odd. Compare:

Natural: Ο καιρός είναι καλός.
Unnatural: Καιρός είναι καλός.

Why does καλός end in -ος?

Adjectives agree with the noun in gender, number, and case. καιρός is masculine, singular, nominative, so the adjective is masculine singular nominative: καλός. Other basic forms:

Feminine: καλή
Neuter: καλό Examples:
Η μέρα είναι καλή. (The day is good.)
Το φαγητό είναι καλό. (The food is good.)

Can I move σήμερα to another position?

Yes. σήμερα (today) is an adverb and is flexible:

Ο καιρός είναι καλός σήμερα. (neutral)
Σήμερα ο καιρός είναι καλός. (slight focus on “today”)
Ο καιρός σήμερα είναι καλός. (focus on “the weather today”) All are fine; position changes emphasis, not meaning.

How do I ask “Is the weather good today?” and what’s the question mark in Greek?

Use inversion or just intonation:

Είναι καλός ο καιρός σήμερα;
Ο καιρός είναι καλός σήμερα; Greek uses a semicolon (;) as its question mark. So the sentence ends with “;” not “?”.

How do I make it negative: “The weather is not good today”?

Place δεν before the verb:

Ο καιρός δεν είναι καλός σήμερα. Spoken Greek may reduce δεν to δε, but in writing δεν is standard.

How do I pronounce the sentence?

Approximate guide with stress in CAPS:

Ο: “o” like “o” in “for” (short), [o]
καιρός: keh-ROS, [keˈɾos] (αι = “e”; ρ is a tapped r)
είναι: EE-neh, [ˈine] (ει = “i”)
καλός: ka-LOS, [kaˈlos]
σήμερα: SEE-meh-ra, [ˈsimeɾa] (η = “i”) Tip: Greek stress is marked by the accent (´) on the stressed syllable in writing.

Is the και in καιρός the same as the word και meaning “and”?

No. It’s just part of the word καιρός (weather, season, opportune time). It has nothing to do with the conjunction και (“and”).

Could I say ωραίος instead of καλός? Any nuance?

Yes:

Ο καιρός είναι ωραίος σήμερα. Both mean the weather is pleasant. ωραίος often feels a bit more “nice/beautiful,” while καλός is a straightforward “good/fine.” Both are common.

Are there other natural ways to express this without saying “the weather is …”?

Very common alternatives:

Κάνει καλό καιρό σήμερα. (It’s making good weather today.)
Έχει καλό καιρό σήμερα. (It has good weather today.) Also:
Έχει ήλιο. (It’s sunny.)
Έχει λιακάδα. (It’s bright/sunny.)
Έχει ζέστη / κάνει ζέστη. (It’s hot.)
Έχει κρύο / κάνει κρύο. (It’s cold.)

Why not say Ο καιρός είναι καλά σήμερα?

καλά is an adverb (“well”), while you need an adjective to describe the noun καιρός. So use καλός. Use καλά to describe how someone is doing: Είμαι καλά (I’m well).

Where does “very” go? How do I say “The weather is very good today”?

Use πολύ before the adjective:

Ο καιρός είναι πολύ καλός σήμερα.
Ο καιρός σήμερα είναι πολύ ωραίος.

Why does Ο have no accent? Don’t Greek words have written stress?

Monosyllabic function words like the article ο/η/το are written without an accent (tonos). In this sentence, Ο is just the capitalized article “the,” so no accent is written on it.

Could I say Ο καλός καιρός instead? What’s the difference from Ο καιρός είναι καλός?

Ο καλός καιρός = “the good weather” (adjective in attributive position, describes a specific/generic kind of weather as a noun phrase).
Ο καιρός είναι καλός = “the weather is good” (adjective in predicative position after “to be”). Both are correct but used in different structures.

What case is καιρός here?

Nominative singular, because it’s the subject. Basic forms you’ll meet:

Nominative: ο καιρός (the weather)
Genitive: του καιρού (of the weather)
Accusative: τον καιρό (the weather, as object) Example: Τι καιρό κάνει σήμερα; (What’s the weather like today?) — here καιρό is accusative.