De vijfde les begint.

It has a very simple structure:

De vijfde les = the subject
begint = the finite verb

So the sentence is a normal Dutch main clause with subject + verb:

De vijfde les begint.

Because les is a de-word in Dutch.

Dutch singular nouns take either de or het as the definite article, and you have to learn which one each noun uses. Since les is common gender, it takes de:

de les = the lesson

If it were a het-word, you would get het instead.

Vijfde is the ordinal number fifth.

Compare:

vijf = five
vijfde = fifth

So de vijfde les means the fifth lesson.

Because vijfde is simply the correct Dutch ordinal form of vijf.

Dutch ordinals are not all formed with one single ending. For example:

eerste = first
tweede = second
derde = third
vierde = fourth
vijfde = fifth
achtste = eighth
twintigste = twentieth

So vijfde is a form you just need to learn as the standard word for fifth.

Yes. Ordinal numbers usually behave like adjectives when they come before a noun.

So in:

de vijfde les

the word vijfde describes les, just as an adjective would.

You see the same pattern in:

de tweede dag = the second day
het derde hoofdstuk = the third chapter

Begint is the present tense form of the verb beginnen, which means to begin or to start.

The stem is begin, and for third-person singular Dutch normally adds -t:

ik begin = I begin
jij begint = you begin
hij/zij begint = he/she begins

Since de vijfde les is a singular subject, begint is the correct form.

It can mean either, depending on context.

Usually les refers to a lesson, teaching period, or class session. So de vijfde les could mean:

the fifth lesson
or
the fifth class period

A related word is klas, but that usually refers more to the group of students or the class as a school group, not the lesson itself.

Yes. In everyday English, begins and starts both work well here.

So this Dutch sentence could match:

The fifth lesson begins.
or
The fifth lesson starts.

The Dutch verb beginnen covers both of those natural English ideas.

In Dutch, yes/no questions usually put the finite verb first.

So:

De vijfde les begint. = The fifth lesson begins.
Begint de vijfde les? = Does the fifth lesson begin?

That verb-first pattern is very common in Dutch questions.

Yes. Dutch often uses the present tense for planned or scheduled events, just like English does.

For example:

De vijfde les begint om twee uur.

That means:

The fifth lesson starts at two o’clock.

So begint does not have to mean only something happening right now.

A rough learner-friendly pronunciation would be:

vijfde ≈ vayf-duh

The ij sound is somewhat like the vowel in English eye, though not exactly the same in all accents.

And:

begint ≈ buh-GINT

Keep in mind that the Dutch g is usually a harsher throat sound than English g, depending on the region. These are only approximations, but they are useful for getting started.