Learn Hardcore Greek: Μετά την καταιγίδα ο ουρανός είναι μπλε και τα σύννεφα φεύγουν. - After the storm the sky is blue and the clouds go away.

Leaving out the article (μετά καταιγίδα) sounds very foreign or old‑fashioned in modern Greek, even though it might feel normal to an English speaker who is used to “after storm,” “after school,” etc. in certain expressions.

Why is την καταιγίδα in the accusative case here?

The noun καταιγίδα (storm) is feminine. Its forms are:

η καταιγίδα – nominative (subject)
την καταιγίδα – accusative (object, or after many prepositions)

The preposition μετά in modern Greek usually takes the accusative when it means after (in time). So:

μετά την καταιγίδα = after the storm (time) → accusative
(older/learned usage) μετά της καταιγίδος = together with the storm or after the storm in very formal/archaic style, using the genitive.

For everyday modern Greek, μετά + accusative is the normal pattern for after (time).

Could we also say Μετά από την καταιγίδα? Is there a difference?

Yes, you can also say:

Μετά από την καταιγίδα ο ουρανός είναι μπλε…

This is very common in spoken Greek and is fully correct.

Nuance:

μετά την καταιγίδα – slightly more concise / a bit more formal or written style.
μετά από την καταιγίδα – very natural in everyday speech; the από doesn’t change the meaning here.

In most everyday contexts, they are interchangeable.

Can I change the word order and say Ο ουρανός είναι μπλε μετά την καταιγίδα?

Yes. Both are correct:

Μετά την καταιγίδα ο ουρανός είναι μπλε…
Ο ουρανός είναι μπλε μετά την καταιγίδα…

The difference is just emphasis:

Starting with Μετά την καταιγίδα emphasizes the time or the contrast with what happened before the storm.
Starting with Ο ουρανός emphasizes the sky as the topic.

Greek word order is relatively flexible, so both sentences are natural.

Why do we use ο ουρανός (with ο) and not just ουρανός?

Greek uses the definite article much more often than English, especially with:

Unique things: ο ήλιος (the sun), η γη (the earth), ο ουρανός (the sky)
General or generic nouns: ο άνθρωπος, το παιδί, etc.

Here, ο ουρανός means the sky in general (the one sky we see), so Greek almost always uses the article:

Ο ουρανός είναι μπλε. = The sky is blue.

Leaving the article out (ουρανός είναι μπλε) would sound unnatural or poetic/elliptical.

Why is μπλε the same form, even though ουρανός is masculine?

Μπλε is one of those adjectives in Greek that are invariable: they do not change for gender, number, or case. It stays μπλε for masculine, feminine, and neuter:

ο ουρανός είναι μπλε – the sky is blue (masc.)
η θάλασσα είναι μπλε – the sea is blue (fem.)
το φόρεμα είναι μπλε – the dress is blue (neut.)

So you don’t say μπλος, μπλη, etc. It’s always μπλε.

Is there any difference between είναι μπλε and είναι γαλάζιος for the sky?

Both can describe the sky, but they have slightly different nuances:

μπλε – basic word for blue; can be any shade.
γαλάζιος / γαλάζια / γαλάζιο – specifically light blue / sky blue.

So:

Ο ουρανός είναι μπλε. – The sky is blue. (neutral)
Ο ουρανός είναι γαλάζιος. – The sky is light blue / sky-blue (often sounds more vivid, clear-sky image).

Grammatically:

είναι μπλε – μπλε is invariable.
είναι γαλάζιος – γαλάζιος agrees with ουρανός (masculine singular nominative).

Why is it τα σύννεφα and not οι σύννεφοι?

Because το σύννεφο (cloud) is:

Neuter gender: το in singular.
Its regular plural is also neuter: τα σύννεφα.

Forms:

Singular: το σύννεφο – the cloud
Plural: τα σύννεφα – the clouds

So the article must match in gender and number:

τα σύννεφα φεύγουν (neuter plural)
not
οι σύννεφοι φεύγουν (this is grammatically wrong).

Could I leave out the article and say σύννεφα φεύγουν?

You can say σύννεφα φεύγουν, but it changes the feel:

τα σύννεφα φεύγουν – the clouds are leaving (we have specific clouds in mind: the ones from that storm).
σύννεφα φεύγουν – clouds (some clouds) are leaving; more indefinite / descriptive / literary.

In normal descriptive sentences after a specific event (the storm), Greek tends to use the definite article: τα σύννεφα.

Why is there no word for “they” before φεύγουν?

In Greek, subject pronouns (like αυτοί, they) are usually dropped, because the verb ending shows the person and number:

φεύγουν = they leave / they are leaving (3rd person plural).

The subject here is already clear from τα σύννεφα:

…και τα σύννεφα φεύγουν.

Adding αυτά (they, neuter plural) would be possible but usually only for emphasis or contrast:

…και αυτά τα σύννεφα φεύγουν. – and these clouds leave too (emphasis).

Does φεύγουν mean “are leaving” or “leave”? Why present tense here?

Greek φεύγουν (present) can cover both English “are leaving” and “leave”, depending on context.

Here, after a storm:

τα σύννεφα φεύγουν is best understood as
→ the clouds are leaving / are moving away (an ongoing process right after the storm).

If you said:

τα σύννεφα έφυγαν – the clouds left / went away (simple past, completed).

So φεύγουν focuses on the process as it’s happening, not on a finished result.

Do I need a comma before και in …είναι μπλε και τα σύννεφα φεύγουν?

No comma is needed here in Greek. The sentence:

Μετά την καταιγίδα ο ουρανός είναι μπλε και τα σύννεφα φεύγουν.

has two clauses joined with και:

ο ουρανός είναι μπλε
τα σύννεφα φεύγουν

Greek normally does not put a comma before και when simply joining two clauses like this. A comma would only appear if there were a special pause or extra phrase that clearly needed separating.