Learn Hardcore Greek: Ο καιρός είναι ωραίος σήμερα.

Greek typically uses the definite article with general nouns when they are the subject. So Ο καιρός (“the weather”) is the natural way to say “the weather” in this context. Saying just Καιρός είναι ωραίος σήμερα sounds odd in standard speech. You do drop the article in expressions like Έχει ωραίο καιρό (“There is nice weather / It has nice weather”).

Why is it ωραίος and not ωραία or ωραίο?

Adjectives agree with the noun in gender, number, and case:

καιρός is masculine, singular, nominative.
Therefore the adjective is masculine, singular, nominative: ωραίος. The common forms of this adjective are:
masculine: ωραίος
feminine: ωραία
neuter: ωραίο Example: Η μέρα είναι ωραία σήμερα. (“The day is nice today.”) Here, μέρα is feminine, so the adjective is ωραία.

Can I use a different adjective, like καλός?

Yes:

Ο καιρός είναι καλός σήμερα. is very common (“The weather is good today.”).
ωραίος often means “nice/pleasant/beautiful” and can sound a bit more “pleasant” or aesthetic.
όμορφος is more “beautiful/handsome”; you’ll more often hear it with μέρα: Όμορφη μέρα σήμερα. Using όμορφος καιρός is possible but sounds more poetic.

Are there other natural ways to say this in Greek?

Yes, very common alternatives:

Έχει καλό/ωραίο καιρό σήμερα.
Κάνει καλό καιρό σήμερα.
Ωραίος καιρός σήμερα. (elliptical, no verb, very natural in speech) Note: In Έχει ωραίο καιρό, the adjective ωραίο is masculine accusative (matching καιρό), and it happens to look the same as the neuter form—this is normal.

Can I change the word order?

Greek word order is flexible. All of these are fine, with small differences in emphasis:

Σήμερα ο καιρός είναι ωραίος. (fronts “today”)
Ο καιρός σήμερα είναι ωραίος. (slight focus on “the weather today”)
Ο καιρός είναι ωραίος σήμερα. (neutral, as given)
Ωραίος ο καιρός σήμερα. / Ωραίος καιρός σήμερα. (focus on “nice”/elliptical, more conversational)

Can I drop the verb είναι?

In full, neutral statements you keep it. But in headlines or casual, elliptical speech you can drop it:

Ωραίος καιρός σήμερα. This works like an exclamation or a clipped statement. In a complete sentence, keep είναι.

How do I pronounce the sentence?

Stress (shown by capital letters) and a simple guide:

Ο: o
καιρός: ke-ROS (ai = “e”; r is tapped)
είναι: EE-neh (ei = “ee”)
ωραίος: o-RE-os (ai = “e”)
σήμερα: SEE-me-ra (stress on the first syllable) Altogether: o ke-ROS EE-neh o-RE-os SEE-me-ra.

How do I turn it into a question: “Is the weather nice today?”

Just change the intonation and use the Greek question mark (which looks like a semicolon):

Ο καιρός είναι ωραίος σήμερα; Greek doesn’t require inversion (you don’t swap word order to make a question).

How do I say “The weather is not nice today.”?

Use δεν before the verb:

Ο καιρός δεν είναι ωραίος σήμερα. You can intensify: Ο καιρός δεν είναι καθόλου ωραίος σήμερα. (“not at all nice”)

Is καιρός always “weather”?

Mostly, yes—but it also means “time/season” in some contexts and idioms:

καιρός είναι να φύγουμε = “it’s time to leave”
καλοκαιρία (good weather) and κακοκαιρία (bad weather) are related words. Also, the plural οι καιροί tends to mean “times/eras” rather than “weathers.”

Why not Το καιρό είναι ωραίο σήμερα?

That’s wrong for two reasons:

καιρός is masculine, so the nominative article is ο, not το.
As the subject, καιρός must be nominative: Ο καιρός, not το καιρό (which would be neuter or accusative forms and incorrect here).

Do I need a comma before or after σήμερα?

No comma is needed in any of the versions shown. A single-word adverb like σήμερα normally doesn’t take a comma when fronted: Σήμερα ο καιρός είναι ωραίος.

Where is the stress in each word, and does it matter?

Yes, stress is phonemic in Greek:

καιρός: stress on -ρός
είναι: stress on -εί-
ωραίος: stress on -ραί-
σήμερα: stress on σή- Putting the stress on the wrong syllable can make a word sound odd or resemble a different word, so it’s worth practicing.