W czerwcu dzieci kończą szkołę i myślą już o wakacjach.

Because after w meaning in, Polish uses the locative case for months.

• basic form: czerwiec = June
• after w: w czerwcu = in June

This pattern is very common:

• w maju = in May
• w lipcu = in July
• w sierpniu = in August

So w czerwcu is not a different word you just memorize separately; it is the locative form of czerwiec.

Dzieci is the plural of dziecko (child).

• dziecko = child
• dzieci = children

It is an irregular plural, so it does not follow a simple pattern like adding -y or -i. In this sentence, dzieci is the subject, so it is in the nominative plural.

Kończą is the 3rd person plural present tense form of kończyć (to finish).

Because the subject is dzieci (children), the verb must also be plural:

• dziecko kończy = the child finishes
• dzieci kończą = the children finish

The ending -ą is very common in 3rd person plural present forms in Polish:

• robią = they do / make
• idą = they go
• myślą = they think

Because szkołę is the accusative singular form of szkoła.

The verb kończyć takes a direct object, and direct objects are often in the accusative in Polish.

• nominative: szkoła
• accusative: szkołę

So:

• Szkoła jest duża. = The school is big.
  (szkoła is the subject)
• Dzieci kończą szkołę. = The children finish school.
  (szkołę is the object)

Because the sentence means that the children are finishing school as an institution / school for the year, not several separate schools.

Polish often uses the singular here, just like English often says:

• Children finish school in June not necessarily
• Children finish schools in June

So szkołę is singular because each child is understood as finishing school, not because there is only one child.

Polish usually does not need subject pronouns like I, you, he, they unless they are needed for emphasis or contrast.

The verb ending already tells you the subject is plural:

• myślą = they think

And since dzieci has already been mentioned, there is no need to repeat a pronoun.

A fuller version would be possible, but it would sound unnecessary in a neutral sentence:

• W czerwcu dzieci kończą szkołę i one myślą już o wakacjach.

That sounds more emphatic than normal.

Because the preposition o after myśleć (to think about) takes the locative case.

• myśleć o kimś / o czymś = to think about someone / something

So:

• basic form: wakacje
• after o: o wakacjach

Also, wakacje is a plural-only noun in Polish. It behaves like a plural word even when English uses a singular-style idea such as the holidays or vacation.

Yes, in normal usage wakacje is a pluralia tantum noun, meaning it is used only in the plural.

So you say:

• wakacje są długie = the holidays/vacation are long
• myślę o wakacjach = I’m thinking about the holidays/vacation

This is similar to how some English nouns are naturally plural in form, even when they refer to one general idea.

Już means already here.

It shows that the children are already thinking about the holidays, even before school is fully over or as it is ending.

In this sentence:

• myślą już o wakacjach

the już naturally sits before the phrase o wakacjach. That placement is very normal.

Polish word order is somewhat flexible, so you may also hear:

• już myślą o wakacjach

The meaning stays very similar, but the focus shifts slightly. The version in your sentence sounds very natural and neutral.

Polish often uses the present tense to describe:

• general truths
• habitual events
• things that regularly happen at a certain time

So this sentence is not necessarily talking about one exact group of children right now. It can mean something like a general fact about what happens in June.

That is why kończą and myślą are in the present tense.

English does something similar in sentences like:

• In June children finish school and start thinking about summer.

This is a question about aspect, which is very important in Polish.

• kończyć = imperfective
• skończyć = perfective

In a general statement like this, kończą szkołę sounds natural because it describes what happens around that time of year in a broad, regular sense.

If you said skończą, it would usually sound more like a future completed action:

• Dzieci skończą szkołę w czerwcu. = The children will finish school in June.

So:

• kończą = a general/habitual description
• skończą = one completed future event

Yes. Polish word order is more flexible than English word order because endings show the grammatical roles.

For example, these are possible:

• W czerwcu dzieci kończą szkołę i myślą już o wakacjach.
• Dzieci kończą szkołę w czerwcu i już myślą o wakacjach.

The basic meaning stays the same, but the emphasis changes.

Your original sentence begins with W czerwcu, which puts the time frame first. That is very natural when you want to set the scene: In June...

A rough pronunciation guide:

• myślą sounds approximately like mish-lon or mish-won, depending on how carefully it is pronounced

A few important points:

• myś-: the ś is a soft sound, somewhat like a very soft sh
• lą: the ą is a nasal vowel; at the end of a word it is often pronounced somewhat like on or om

So do not pronounce it like English my-slay or miss-lah.

A rough guide is:

• czerwcu ≈ CHERF-tsoo or CHERF-tsu

Useful details:

• cz sounds like English ch
• w in Polish sounds like English v
• but in clusters like rwc, the w may sound devoiced, closer to f
• final u sounds like oo

So the word may sound a bit compressed in normal speech. Polish consonant clusters often do that.