Hava kötü; öyleyse evde kalalım.

In Turkish, the verb to be is often left out in the present tense.

So Hava kötü literally looks like weather bad, but it means The weather is bad.

This is completely normal in Turkish:

• Hava kötü. = The weather is bad.
• Ev büyük. = The house is big.
• Ben yorgunum. = I am tired.
  (With pronouns, Turkish often adds a personal ending.)

You could also see Hava kötüdür, but that sounds more formal, general, or emphatic.

Öyleyse means so, then, in that case, or if that’s so.

It connects the first idea to a conclusion:

• Hava kötü → the weather is bad
• öyleyse → so / in that case
• evde kalalım → let’s stay at home

It is a logical connector, similar to:

• so
• therefore
• in that case

A close alternative is o hâlde.

You may see both:

• öyleyse
• öyle ise

In modern everyday writing, öyleyse as one word is very common.

Both mean the same thing here. The combined form often feels smoother and more natural in ordinary usage.

Evde means at home or in the house.

It contains the locative suffix:

• ev = house / home
• -de = in / at

So:

• evde = at home

It is not eve, because kalmak usually goes with a location, not a destination.

Compare:

• evde kalmak = to stay at home
• eve gitmek = to go home

So:

• evde = where you stay
• eve = where you go

Kalalım means let’s stay.

It is a first person plural suggestion form, sometimes called the let’s form or cohortative.

So:

• kalalım = let us stay / let’s stay

In this sentence, it sounds like a suggestion:

• The weather is bad, so let’s stay at home.

It comes from the verb kalmak, meaning to stay or to remain.

Breakdown:

• kal- = verb stem
• -alım = let’s ...

So:

• kal + alım → kalalım = let’s stay

This suffix changes by vowel harmony in other verbs:

• gidelim = let’s go
• oturalım = let’s sit down / let’s sit
• bakalım = let’s see / let’s look

Because Turkish verb endings usually already show the subject.

Kalalım already means let’s stay / let us stay, so biz is unnecessary.

You could say Biz evde kalalım, but then biz adds emphasis, like:

• As for us, let’s stay at home
• We should stay at home

Most of the time, Turkish leaves pronouns out unless they are needed for emphasis or contrast.

Turkish does not have a word exactly like English the.

So hava can mean:

• weather
• the weather

Which one it means depends on context.

In Hava kötü, it naturally means the weather is bad.

Turkish also has no direct equivalent of English a/an in the same way, though bir can sometimes play that role.

Yes, Turkish word order is flexible, although some orders sound more natural than others.

The given sentence:

• Hava kötü; öyleyse evde kalalım.

This is very natural.

You could also hear:

• Hava kötü, evde kalalım öyleyse.
• Öyleyse evde kalalım; hava kötü.
  This changes the emphasis a bit.

In Turkish, word order often shifts for focus, style, or emphasis, but the standard flow here is very clear:

1. state the situation
2. draw the conclusion

The semicolon separates two closely related clauses:

• Hava kötü
• öyleyse evde kalalım

It shows a stronger break than a comma, but the two parts are still tightly connected.

In everyday writing, many people would also use a comma:

• Hava kötü, öyleyse evde kalalım.

Or even split it into two sentences:

• Hava kötü. Öyleyse evde kalalım.

So the semicolon is mainly a punctuation choice, not a grammar requirement.

It is fairly neutral and perfectly normal in both speech and writing.

However, in very casual conversation, people might choose other connectors depending on tone, such as:

• o zaman = then
• o hâlde = in that case / then
• o yüzden = that’s why / so

These are not always exact substitutes, but they can be similar depending on context.

So öyleyse is natural, clear, and slightly logical in tone.

Because kalmak means to stay / remain, which fits the idea of staying at home.

English often uses stay in exactly the same way:

• Let’s stay at home.

Turkish also says:

• evde kalmak = to stay at home

If you used another verb, the meaning would change:

• evde oturmak = to sit at home / to live at home / to stay home, depending on context
• evde olmak = to be at home

So kalmak is the best match for the idea of remaining at home instead of going out.

A few natural alternatives are:

• Hava kötü, evde kalalım.
  Very natural and simple: The weather is bad, let’s stay home.

• Hava kötü; o hâlde evde kalalım.
  Slightly more formal or structured: The weather is bad; in that case, let’s stay at home.

• Hava kötü olduğu için evde kalalım.
  More explicitly causal: Since the weather is bad, let’s stay at home.

The original sentence is already correct and natural. It simply uses a connector that clearly shows reason → conclusion.