Ο καιρός είναι ασυνήθιστος σήμερα.

Ο is the definite article the in the masculine singular nominative form.

Greek usually uses the article with nouns in normal sentences, so ο καιρός is the natural way to say the weather. If you dropped the article and said just καιρός, it would sound less like an ordinary full sentence and more like a headline, note, or dictionary style.

No. Καιρός is a very common Greek word with more than one meaning. Depending on context, it can mean:

• weather
• time
• season
• the right moment / opportunity

In this sentence, because of σήμερα and the general context, it clearly means weather.

It is in the nominative singular.

That is the form used for the subject of the sentence, and here ο καιρός is the subject of είναι.

Very roughly:

• ο καιρός = nominative, the weather
• τον καιρό = accusative
• του καιρού = genitive

So in this sentence, ο καιρός is the thing being described as unusual.

Because Greek adjectives must agree with the noun they describe in:

• gender
• number
• case

Here, καιρός is masculine singular nominative, so the adjective also appears in the masculine singular nominative form:

• ασυνήθιστος = masculine
• ασυνήθιστη = feminine
• ασυνήθιστο = neuter

So ο καιρός είναι ασυνήθιστος is grammatically matched all the way through.

Είναι is the present-tense form of είμαι, meaning to be.

In this sentence, it means is.

A useful thing to know is that είναι is used for both:

• he/she/it is
• they are

So the same form works for both third person singular and third person plural.

Greek does not use a dummy subject like English often does.

In English, you can say It is unusual today. In Greek, you normally do not insert a meaningless it. Instead, you say the real subject, such as:

• Ο καιρός είναι ασυνήθιστος σήμερα.

More generally, Greek often leaves out subject pronouns when they are not needed, because the verb form and context usually make things clear.

Yes. Greek word order is more flexible than English word order.

You can also say:

• Σήμερα ο καιρός είναι ασυνήθιστος.
• Ο καιρός σήμερα είναι ασυνήθιστος.

All of these are possible. The version Ο καιρός είναι ασυνήθιστος σήμερα is very natural and neutral.

Changing the position can slightly change the emphasis:

• Σήμερα... gives more emphasis to today
• putting σήμερα at the end can sound more like an afterthought or final time setting

A rough English-style pronunciation is:

o keh-ROS EE-ne a-see-NEE-thee-stos SEE-me-ra

A few helpful notes:

• καιρός is stressed on the last syllable: keh-ROS
• είναι is pronounced roughly EE-ne
• θ sounds like th in think
• σήμερα is stressed on the first syllable: SEE-me-ra

The accent marks in Greek show you which syllable gets the stress.

Yes. Ασυνήθιστος is a good, normal word for unusual or out of the ordinary.

It is often more precise than words like περίεργος, which can mean:

• strange
• weird
• odd
• sometimes even curious

So if you mean that the weather is not typical, ασυνήθιστος is a very good choice.

You could, but it would not mean exactly the same thing.

• Ο καιρός είναι ασυνήθιστος σήμερα = The weather is unusual today
• Ο καιρός είναι περίεργος σήμερα = more like The weather is strange/weird today

So ασυνήθιστος is better if you want the idea of not normal, while περίεργος sounds a bit stronger or more subjective.