Czerwiec bywa ciepły, ale wieczorem bywa jeszcze zimno.

Bywa is the 3rd person singular of bywać, which often means to be sometimes, to tend to be, or to be on some occasions.

So:

• Czerwiec jest ciepły = June is warm
  This sounds more like a general statement or description.
• Czerwiec bywa ciepły = June can be warm / June is sometimes warm
  This suggests it is not always warm.

In this sentence, bywa gives the idea of something that happens from time to time, not something guaranteed.

It is used once in each clause:

• Czerwiec bywa ciepły
• ale wieczorem bywa jeszcze zimno

This makes the sentence balanced and keeps the same idea in both parts: both the warmth of June and the evening cold are being presented as things that sometimes happen.

You could say:

• Czerwiec bywa ciepły, ale wieczorem jest jeszcze zimno.

That is also possible, but it changes the nuance a bit. Then the first part means June can be warm, while the second part sounds more like a straightforward fact: but in the evening it is still cold.

Because ciepły is an adjective describing czerwiec.

• czerwiec = June
• ciepły = warm

Since czerwiec is a masculine singular noun, the adjective must agree with it:

• ciepły czerwiec = a warm June

So in Czerwiec bywa ciepły, the adjective is in the masculine singular form because it matches czerwiec.

Because zimno here does not describe a noun. It describes the weather/temperature situation in an impersonal way.

Polish often uses words like:

• ciepło = warm
• zimno = cold
• gorąco = hot

in sentences like:

• Jest zimno. = It is cold.
• Bywa zimno. = It can be cold.

So:

• czerwiec bywa ciepły = the month June is warm
• wieczorem bywa jeszcze zimno = in the evening, it is still cold

The first uses an adjective agreeing with a noun.
The second uses an impersonal temperature expression.

Wieczorem is a very common Polish way to say in the evening.

It is an instrumental form of wieczór and works as a fixed time expression.

Compare:

• rano = in the morning
• wieczorem = in the evening
• nocą = at night / during the night

So:

• wieczorem bywa jeszcze zimno = in the evening it is still cold

You will hear wieczorem much more naturally than w wieczór, which would usually not be used in this general time meaning.

Here jeszcze means still.

So:

• wieczorem bywa jeszcze zimno = in the evening it is still cold

It suggests that even though June may already be warm, the evening cold has not disappeared yet.

Depending on context, jeszcze can also mean yet, more, or another, but here still is the best match.

Because Polish often uses impersonal weather expressions without a real subject.

In English you say:

• It is cold

But the it does not really refer to anything specific. It is just a grammatical subject.

In Polish, you usually do not need that kind of dummy subject:

• Jest zimno
• Bywa zimno

So wieczorem bywa jeszcze zimno is completely normal even though there is no word corresponding to English it.

Because Czerwiec here means June as the month itself, treated as the subject of the sentence:

• Czerwiec bywa ciepły = June can be warm

If you say w czerwcu, that means in June, and the sentence structure changes:

• W czerwcu bywa ciepło = It can be warm in June

Both are correct, but they are built differently:

• Czerwiec bywa ciepły → June is the subject, so you use an adjective agreeing with czerwiec
• W czerwcu bywa ciepło → in June is a time phrase, and ciepło is used impersonally

The sentence is natural as written:

• Czerwiec bywa ciepły, ale wieczorem bywa jeszcze zimno.

Polish word order is more flexible than English, but changing it can shift emphasis.

For example:

• Wieczorem bywa jeszcze zimno.
  Neutral and natural.
• Jeszcze wieczorem bywa zimno.
  Slightly stronger focus on still in the evening.
• Zimno bywa jeszcze wieczorem.
  Grammatically possible, but less natural in everyday speech.

So the original order is a very good standard pattern for a learner.

Yes, that is grammatically correct, but the meaning changes a little.

• jest = a more direct statement of fact
• bywa = something that happens sometimes / can happen

So:

• Czerwiec jest ciepły... sounds like a stronger generalization
• Czerwiec bywa ciepły... sounds more cautious and realistic

In a sentence about weather, bywa is often very natural because weather is not constant.

Yes, very much so.

Polish often talks about weather in two common patterns:

1. Month/season + adjective

   • Maj jest chłodny.
   • Luty bywa mroźny.

2. Impersonal weather expression

   • Jest zimno.
   • Bywa ciepło.
   • Wieczorem jest chłodno.

This sentence uses both patterns together, which is why it is especially useful for learners:

• Czerwiec bywa ciepły
• wieczorem bywa jeszcze zimno