Υποθέτω ότι η πρόγνωση για αύριο δεν είναι καλή, γιατί έχει ήδη πολλή συννεφιά.

Υποθέτω means I suppose or I assume.

It is the 1st person singular, present tense form of υποθέτω. So the speaker is giving their current opinion or assumption.

In this sentence, it sounds natural and a bit tentative, like: I suppose / I assume the forecast for tomorrow isn’t good...

It is not as strong as a firm statement of fact.

ότι introduces a content clause, the equivalent of English that:

Υποθέτω ότι... = I suppose that...

In English, that is often optional, but in Greek it is very common to include ότι.

You can often also use πως in the same kind of sentence: Υποθέτω πως η πρόγνωση...

Both are natural here.

This is a very common learner question because they look almost the same.

• ότι = that
• ό,τι = whatever / anything that / what

So in this sentence, ότι is correct because it introduces the clause after Υποθέτω:

Υποθέτω ότι η πρόγνωση...
= I suppose that the forecast...

If you wrote ό,τι, the meaning would change completely.

πρόγνωση means forecast, and it is a feminine noun. That is why its article is η.

So:

• η πρόγνωση = the forecast

Greek uses the definite article very often, including in places where English also uses the. Here it refers to a specific forecast, namely the forecast for tomorrow.

για here means for or regarding, and αύριο means tomorrow.

So:

• η πρόγνωση για αύριο = the forecast for tomorrow

This is a very common Greek pattern:

• σχέδια για αύριο = plans for tomorrow
• το πρόγραμμα για σήμερα = the schedule for today

Also, αύριο is commonly used without an article in this kind of expression.

δεν is the normal negation used with indicative verbs in Modern Greek.

So:

• είναι = is
• δεν είναι = is not

In Greek, δεν normally goes directly before the verb:

• δεν είναι
• δεν ξέρω
• δεν βλέπω

So η πρόγνωση ... δεν είναι καλή literally means the forecast ... is not good.

Because καλή must agree with η πρόγνωση, which is feminine singular.

The adjective καλός changes form depending on gender:

• καλός = masculine
• καλή = feminine
• καλό = neuter

Since πρόγνωση is feminine, Greek says:

η πρόγνωση είναι καλή
= the forecast is good

Even though καλή comes after είναι, it still agrees with the noun it describes.

Yes. γιατί can mean both:

• because
• why

In this sentence, it clearly means because:

..., γιατί έχει ήδη πολλή συννεφιά.
= ..., because it is already very cloudy / because there is already a lot of cloudiness.

When it means why, it usually appears in a question:

Γιατί έχει συννεφιά;
= Why is it cloudy?

Context and punctuation normally make the meaning clear.

The comma separates the main clause from the clause giving the reason.

So the structure is:

• main statement: Υποθέτω ότι η πρόγνωση για αύριο δεν είναι καλή
• reason: γιατί έχει ήδη πολλή συννεφιά

This comma is natural in Greek when γιατί means because and introduces an explanatory clause.

This is a very useful Greek pattern.

Literally, έχει ήδη πολλή συννεφιά is something like:

it has already a lot of cloudiness

But in natural English, that becomes:

• it’s already very cloudy
• there is already a lot of cloud cover
• there are already lots of clouds

Greek often uses έχει in an impersonal way for weather or general conditions:

• έχει ζέστη = it’s hot
• έχει κρύο = it’s cold
• έχει ήλιο = it’s sunny
• έχει συννεφιά = it’s cloudy

So there is no explicit subject like English it. Greek simply uses έχει idiomatically.

ήδη means already.

In this sentence:

έχει ήδη πολλή συννεφιά
= it is already very cloudy / there is already a lot of cloudiness

Its placement is natural because it modifies the whole situation and especially highlights that the cloudiness has appeared by now.

Greek adverbs like ήδη can sometimes move around, but this position is very normal and clear.

Because πολλή here is an adjective, not an adverb.

συννεφιά is a feminine singular noun, so the adjective must agree with it:

• πολύς = masculine
• πολλή = feminine
• πολύ = neuter

So:

• πολλή συννεφιά = a lot of cloudiness

By contrast, πολύ is also the form used as an adverb meaning very:

• πολύ καλός = very good
• πολύ όμορφη = very beautiful

So compare:

• πολλή συννεφιά = a lot of cloudiness
• πολύ καλή = very good

That distinction is extremely important in Greek.

συννεφιά means cloudiness, overcast conditions, or simply cloudy weather.

It is an uncountable noun, so Greek treats it like a mass noun here:

• έχει συννεφιά = it’s cloudy
• έχει πολλή συννεφιά = it’s very cloudy / there’s a lot of cloud cover

It is related to σύννεφο, which means cloud.

So:

• σύννεφο = cloud
• συννεφιά = cloudiness / cloudy weather

Yes. Greek has several natural ways to express this idea.

For example:

• είναι ήδη πολύ συννεφιασμένος ο καιρός = the weather is already very cloudy
• ο καιρός έχει ήδη χαλάσει = the weather has already turned bad
• υπάρχουν ήδη πολλά σύννεφα = there are already a lot of clouds

But έχει ήδη πολλή συννεφιά is very natural everyday Greek and is a common weather expression.