Η πρόγνωση λέει ότι αύριο θα έχει καύσωνα, γι’ αυτό θέλω να μείνω σπίτι το μεσημέρι.

Η πρόγνωση means the forecast.

• πρόγνωση = forecast, prediction
• η = the feminine singular article

Greek uses the definite article more often than English, and here it works exactly like English the: the forecast.

Yes. Η πρόγνωση λέει... literally means The forecast says...

This is very natural in Greek, just as English can say The forecast says it will be hot tomorrow. Greek often uses λέω in this way for things like forecasts, signs, instructions, and reports.

• λέει = it says
• It is 3rd person singular present, matching η πρόγνωση

ότι means that and introduces a clause:

• Η πρόγνωση λέει ότι...
• The forecast says that...

In English, that is often optional. Greek can also sometimes omit ότι in everyday speech, but using it is completely normal and clear.

αύριο means tomorrow, and it is placed early because it sets the time for what follows:

• ότι αύριο θα έχει καύσωνα
• that tomorrow there will be a heatwave

Greek word order is more flexible than English. Putting αύριο before the verb is very natural. You could also hear other word orders depending on emphasis, but this one is standard.

θα έχει is a future form.

• θα = future particle
• έχει = has

So literally it looks like will have, but here the meaning is closer to:

• there will be a heatwave
• it will be extremely hot

Greek forms the future with θα + verb.

Because Greek often uses έχει in weather-style expressions where English uses there is / there will be or just it is / it will be.

So:

• θα έχει καύσωνα = there will be a heatwave
• more naturally in English: it’s going to be extremely hot

This is a normal Greek way to talk about weather conditions. It is idiomatic, not something you should translate word-for-word every time.

καύσωνα is the accusative singular form of ο καύσωνας.

Why accusative? Because it is the object of έχει:

• έχει καύσωνα = it has a heatwave / there is a heatwave

So the forms are:

• nominative: ο καύσωνας
• accusative: τον καύσωνα

But after έχει, Greek usually does not need the article here, so you get simply καύσωνα.

γι’ αυτό means for that, and in natural English here it means therefore, so, or that’s why.

• γι’ αυτό θέλω να μείνω σπίτι
• that’s why I want to stay home

The apostrophe shows a shortened form of για αυτό. In everyday Greek, this contraction is very common:

• για αυτό → γι’ αυτό

In Modern Greek, after θέλω when you want to say I want to do something, you use:

• θέλω να + verb

So:

• θέλω να μείνω = I want to stay

You cannot say θέλω μένω for this meaning.

The particle να introduces the following verb in the subjunctive-type construction, which is extremely common in Greek.

This is a very common learner question.

• να μείνω uses the aorist stem
• να μένω uses the present stem

Here να μείνω is more natural because it refers to a single, specific action or decision: to stay home tomorrow at midday.

Compare:

• θέλω να μείνω σπίτι το μεσημέρι = I want to stay home at midday tomorrow
• θέλω να μένω σπίτι τα μεσημέρια = I want to stay home at middays / as a habit

So in this sentence, να μείνω fits the one-time situation.

Because σπίτι can work almost like an adverb meaning home / at home.

So:

• μένω σπίτι = I stay home / I stay at home

This is very common and natural Greek.

You can also say μένω στο σπίτι, which is also correct, but:

• σπίτι is shorter and very idiomatic
• στο σπίτι can sound a bit more explicit

Both are possible, but μένω σπίτι is extremely common.

Greek often uses the definite article in time expressions:

• το πρωί = in the morning
• το μεσημέρι = at noon / in the afternoon depending on context
• το βράδυ = in the evening
• τη νύχτα = at night

So το μεσημέρι is a normal fixed time expression. The article is not optional here in standard Greek.

It can cover the midday period, and depending on context it may be understood as:

• at noon
• around lunchtime
• in the midday heat

In this sentence, because of καύσωνα, it strongly suggests during the hottest part of the day, so English might naturally render it as at midday or during the afternoon heat.

Yes, it is very natural:

• Η πρόγνωση λέει ότι αύριο θα έχει καύσωνα, γι’ αυτό θέλω να μείνω σπίτι το μεσημέρι.

Greek word order is flexible, but this sentence is neutral and standard. Parts could move for emphasis, for example:

• Αύριο, λέει η πρόγνωση, θα έχει καύσωνα...
• Γι’ αυτό το μεσημέρι θέλω να μείνω σπίτι.

But the original version is probably what a learner should remember as the safest, most natural pattern.

Yes, several parts are worth learning as set phrases:

• Η πρόγνωση λέει ότι... = The forecast says that...
• θα έχει καύσωνα = there will be a heatwave / it will be extremely hot
• γι’ αυτό = that’s why / so / therefore
• θέλω να μείνω σπίτι = I want to stay home
• το μεσημέρι = at midday / in the middle of the day

If you learn these chunks, the whole sentence becomes much easier to produce naturally.