De negentiende les eindigt.

Because les is a de-word in Dutch. Its singular definite article is de.

• de les = the lesson
• plural: de lessen = the lessons

So in this sentence, de is simply the correct article for les.

Negentiende is an ordinal number: it means nineteenth.

Compare:

• negentien = nineteen
• negentiende = nineteenth

So de negentiende les means the nineteenth lesson.

Dutch usually forms ordinal numbers by changing the cardinal number into an ordinal form, often with -de or -ste.

Here:

• negentien = nineteen
• negentiende = nineteenth

This is just the standard ordinal form you have to learn. English does something similar with nineteen → nineteenth.

It can mean either, depending on context.

Common meanings of les include:

• lesson
• class
• teaching period
• instruction

So de negentiende les could be understood as the nineteenth lesson or the nineteenth class/session, depending on the situation.

Because eindigen is the infinitive (to end), while eindigt is the present tense form used with a singular subject like de negentiende les.

Conjugation of eindigen in the present tense:

• ik eindig = I end
• jij eindigt / eindig je = you end
• hij/zij/het eindigt = he/she/it ends

Since de negentiende les is singular, Dutch uses eindigt.

The subject is de negentiende les.

That whole noun phrase is what is doing the action of eindigt.

So the structure is:

• De negentiende les = subject
• eindigt = verb

No, this is a very basic Dutch main clause with normal word order:

• Subject + finite verb

So:

• De negentiende les = subject
• eindigt = finite verb

This is similar to English: The nineteenth lesson ends.

A useful thing to know, though, is that Dutch main clauses follow the verb-second rule. In this sentence, the subject comes first, so the verb naturally comes second.

Yes, in many contexts the Dutch simple present can cover both ideas.

So De negentiende les eindigt can mean something like:

• The nineteenth lesson ends
• The nineteenth lesson is ending

The exact English translation depends on context.

Two parts often feel difficult for English speakers:

• ij sounds somewhat like the vowel in line or time, though not exactly the same
• g is the harsh Dutch g, made in the back of the throat in many accents

So eindigt may feel tricky at first. The important thing is to notice that it is roughly:

• ein- with an eye-like sound
• -digt with a Dutch guttural g

Even if your pronunciation is not perfect yet, Dutch speakers will usually understand you if the word is otherwise clear.

No. Eindigen is a normal inseparable verb.

So you say:

• De les eindigt.

You do not split it into parts the way you would with a separable verb such as opstaan → ik sta op.

That makes eindigen a bit easier to use.