In juni begint de zomer.

Because Dutch often allows you to put a time expression first if you want to emphasize it.

So both of these are correct:

• In juni begint de zomer.
• De zomer begint in juni.

The difference is mostly about focus. Starting with In juni highlights the time.

Also, Dutch main clauses follow the V2 rule: the conjugated verb must be in the second position. So if In juni comes first, the verb begint must come next.

Begint is the correct conjugated form of the verb beginnen for a singular subject.

The subject here is de zomer, which is third-person singular, so:

• ik begin = I begin
• jij begint / begin jij = you begin
• hij/zij/het begint = he/she/it begins
• de zomer begint = the summer begins

So begint matches de zomer.

This happens because of normal Dutch word order in a main clause.

In Dutch, the finite verb usually comes second. If the sentence begins with something other than the subject, the subject comes after the verb.

So:

• De zomer begint in juni.
  Subject first, then verb.

But if you move In juni to the front:

• In juni begint de zomer.

Now In juni takes the first position, so the verb begint must come second, and the subject de zomer comes after it.

This is called inversion.

Because zomer is normally used as a noun with an article in this kind of sentence.

In Dutch, the names of the seasons are usually:

• de lente
• de zomer
• de herfst
• de winter

So zomer is a de-word, and here it appears as de zomer.

Dutch also often uses the definite article in general statements where English may or may not focus on it in the same way.

Yes. For months, Dutch normally uses in.

Examples:

• in januari
• in april
• in juni
• in december

This works much like English in June, in January, etc.

Also notice that Dutch usually does not use an article before month names:

• in juni
• not in de juni

Because in Dutch, names of months are normally written with a lowercase letter.

So Dutch writes:

• januari
• februari
• juni

The same is true for seasons:

• zomer
• winter

This is different from English, where June is capitalized.

Of course, if the word comes at the beginning of a sentence, it gets a capital because it starts the sentence.

Yes, absolutely. That is also correct and very natural.

Compare:

• In juni begint de zomer.
• De zomer begint in juni.

They mean the same thing, but the emphasis is slightly different:

• In juni begint de zomer. → emphasis on when
• De zomer begint in juni. → more neutral/basic order

A native English speaker often notices that Dutch j is not pronounced like English j in June.

In Dutch:

• j sounds like English y in yes
• u in juni is a Dutch vowel that does not exist in standard English

So juni sounds roughly like YOO-nee, but with a more rounded vowel than English oo.

Very rough guide:

• ju ≈ a rounded yu
• ni ≈ nee

And begint is roughly buh-GINT with the stress on the second syllable.

Yes, very closely.

Beginnen means to begin, and in this sentence it is used intransitively:

• De zomer begint. = Summer begins.

Dutch can also use it with other kinds of complements:

• De les begint om negen uur. = The lesson begins at nine.
• Ik begin morgen. = I begin tomorrow / I’m starting tomorrow.

So in this sentence, begint works very much like English begins.

No. Normally you do not put a comma after a short introductory phrase like In juni in Dutch.

So the normal sentence is:

• In juni begint de zomer.

A comma would usually look unnatural here unless there were a special stylistic reason.