Het nummer is dertien.

Because nummer is a het-word in Dutch. That means its singular definite article is het.

• het nummer = the number
• de nummers = the numbers

This is something you usually have to learn with each noun, because grammatical gender in Dutch is not always predictable.

Nummer is a noun. In this sentence, it is a singular noun.

A few common meanings of nummer are:

• number
• song / track
• issue of a magazine
• sometimes a sports event or act

In Het nummer is dertien, it simply means number.

Is is the present-tense singular form of zijn, which means to be.

The subject here is het nummer, which is singular, so Dutch uses is:

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

So:

• Het nummer is dertien. = The number is thirteen.

Because dertien is being used as a number after the verb zijn. In this kind of sentence, Dutch normally does not use an article.

So you say:

• Het nummer is dertien.

not:

• Het nummer is de dertien.

The word dertien is functioning as the complement of the sentence, not as a noun with an article.

A good approximate pronunciation is der-TEEN.

A few useful notes:

• The stress is on the second syllable: der-TIEN
• The r can sound different depending on region
• The ie sounds like the vowel in English see

Learners often mix up:

• dertien = thirteen
• dertig = thirty

So it is worth practicing those separately.

Yes. This is a standard main-clause word order:

• Het nummer = subject
• is = verb
• dertien = complement

So the pattern is basically:

subject + verb + complement

That is very normal in simple Dutch statements.

You usually make a yes/no question by putting the verb first:

• Is het nummer dertien?

That means Is the number thirteen?

This is a very common pattern in Dutch:

• statement: Het nummer is dertien.
• question: Is het nummer dertien?

Normally, no. In a full standard sentence, you need the article:

• Het nummer is dertien.

Without het, it sounds incomplete in ordinary Dutch.

However, in special contexts, such as labels, notes, or very short spoken answers, you might hear something shorter like:

• Nummer dertien.

But that is not the normal full-sentence version.

Not always. Dutch nummer can also mean song or track.

For example:

• Dat is een mooi nummer. = That is a nice song.

So context matters. In Het nummer is dertien, the meaning is clearly about a number, but you should know that nummer has more than one common use.

Yes. Dutch numbers from 13 to 19 usually end in -tien:

• dertien = 13
• veertien = 14
• vijftien = 15
• zestien = 16
• zeventien = 17
• achttien = 18
• negentien = 19

So once you learn the pattern, these become easier to recognize.

In this sentence, het is the definite article, so it means the, not it.

• het nummer = the number

Dutch het can also be a pronoun meaning it, but that is a different use.

Compare:

• Het nummer is dertien. = The number is thirteen.
• Het is dertien. = It is thirteen.

Same word, different grammatical function.