September is koud.

Word for word:

• September = September
• is = is
• koud = cold

So the structure is very close to English: September is cold.

Because Dutch and English are closely related languages, and some very common words still look alike.

Here, is is the third-person singular present form of zijn = to be.

A few forms of zijn are:

• ik ben = I am
• jij bent = you are
• hij/zij/het is = he/she/it is

So in September is koud, is matches the singular subject September.

Because September is the name of a month, and month names are treated like proper names.

So Dutch normally says:

• September is koud
• In september begint de herfst

Not:

• de September
• het September

That is the same basic idea as in English: we say September, not the September, in ordinary use.

Because koud is a predicate adjective here: it comes after is and describes the subject.

After verbs like zijn = to be, Dutch adjectives usually stay in the basic form:

• Het water is koud
• September is koud
• De soep is warm

But before a noun, the adjective often gets -e:

• de koude maand
• een koude septembermaand

So:

• September is koud = correct
• September is koude = incorrect

Yes, it is grammatical and understandable.

However, depending on what you mean, Dutch speakers might more often say:

• In september is het koud. = It is cold in September.
• September is een koude maand. = September is a cold month.

So September is koud is fine, but it can sound a little direct or simplified. It is the kind of sentence you often see in learning materials because the grammar is clear.

Yes. In normal Dutch, month names are usually lowercase:

• januari
• februari
• september

But at the beginning of a sentence, the first word gets a capital letter, so:

• September is koud.

If the word appears later in the sentence, it is normally lowercase:

• In september is het koud.

Koud is pronounced roughly like kowt, with:

• ou sounding a bit like the vowel in cow
• the final d often sounding very close to t in actual pronunciation

So for an English speaker, koud is approximately:

• kowt

Not an exact English sound, but that is a useful approximation.

A rough English-friendly pronunciation is:

• sep-TEM-ber is kowt

A few notes:

• September is very similar to English September
• is is short, like English is
• koud sounds roughly like kowt

Yes, and that is often the more natural sentence.

• In september is het koud. = It is cold in September.

The word order changes because Dutch uses verb-second word order in main clauses. That means the conjugated verb usually comes in the second position.

So:

• Het is koud in september.
• In september is het koud.

Both are correct. When In september is moved to the front, the verb is still stays in second position, so het comes after the verb.

Yes, absolutely.

That sentence means:

• September is a cold month

This version can sound a bit more natural if you are describing the month in general. Compare:

• September is koud. = September is cold.
• September is een koude maand. = September is a cold month.
• In september is het koud. = It is cold in September.

All three are correct, but they are used slightly differently.