De zestiende les begint.

Because les is a de-word in Dutch: de les.

In Dutch, singular nouns usually take either de or het, and you have to learn which one goes with each noun. Since les is a de-word, anything that belongs with it also follows the de-word pattern.

So:

• de les = the lesson
• de zestiende les = the sixteenth lesson

Zestiende is the ordinal form of zestien.

• zestien = sixteen
• zestiende = sixteenth

Dutch uses ordinal numbers much like English:

• een → eerste = first
• twee → tweede = second
• drie → derde = third
• vier → vierde = fourth
• zestien → zestiende = sixteenth

So de zestiende les literally means the sixteenth lesson.

Because it is being used before a noun: les.

In Dutch, adjectives and ordinal numbers usually take an -e ending when they come before a noun, especially with de-words.

So you get:

• de eerste les
• de tweede les
• de zestiende les

This -e is very normal in Dutch. You will see the same pattern with regular adjectives:

• de mooie les = the nice lesson
• de moeilijke les = the difficult lesson

Because the subject is singular: de zestiende les.

The infinitive is beginnen = to begin.
But in a present-tense sentence with a singular subject, Dutch conjugates the verb:

• ik begin = I begin
• jij begint = you begin
• de les begint = the lesson begins

So:

• De zestiende les begint. = The sixteenth lesson begins / is beginning.

The subject is de zestiende les.

That is the thing doing the action of begint.

So the structure is:

• De zestiende les = subject
• begint = verb

This is a very straightforward Dutch main clause.

Yes, in this sentence it is very similar.

Dutch main clauses normally follow the verb-second rule, and when the subject comes first, the sentence often looks like English:

• De zestiende les begint.
• The sixteenth lesson begins.

So here the order is:

Subject + verb

This sentence is simple and natural Dutch.

Yes. In many cases, the Dutch present tense can match either English simple present or present progressive, depending on context.

So:

• De zestiende les begint. can mean
  
  • The sixteenth lesson begins.
  • The sixteenth lesson is beginning.

Dutch often does not need a separate is ...-ing form the way English does.

A rough English-friendly guide is:

ZES-teen-duh

A few pronunciation notes:

• zes sounds roughly like English zess
• tien sounds like teen, but with a cleaner vowel
• the final -de is a light duh sound

So the whole word is approximately:

ZES-teen-duh

The stress is on the first syllable: ZES.

A rough guide is:

buh-GINT

Notes:

• The be- at the start is usually unstressed, like a soft buh
• The stress falls on gint
• The Dutch g is a throat sound that does not exist in standard English

So an approximate pronunciation for learners is buh-GINT, but with a Dutch g.

Because Dutch does not capitalize all nouns.

Unlike German, Dutch only capitalizes:

• the first word of a sentence
• proper names
• some special cases such as days in titles or formal headings, depending on style

So in normal writing:

• De zestiende les begint.

Only De is capitalized because it starts the sentence.

Not in the same standard way.

If you want to say the sixteenth lesson, the normal form is:

• de zestiende les

Dutch usually uses the ordinal number before the noun, just like English:

• the sixteenth lesson
• de zestiende les

A form like les zestien might be understood in a very specific organizational context, like lesson 16 as a label, but it is not the normal phrasing of this sentence.

The plural of les is lessen.

So:

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

If you made the sentence plural, the verb would also change:

• De zestiende les begint. = The sixteenth lesson begins.
• De lessen beginnen. = The lessons begin.

Notice:

• singular: begint
• plural: beginnen

It is neutral and completely standard.

There is nothing especially formal or informal about it. It is the kind of sentence you might see in:

• a textbook
• a course schedule
• a classroom context
• a story describing events in order

So it is good basic, natural Dutch.