Dit is de derde les.

Both dit and deze can mean this, but they are used differently.

• dit is a demonstrative pronoun: it stands on its own.
  • Dit is de derde les. = This is the third lesson.
• deze is a demonstrative determiner: it comes before a noun.
  • Deze les is moeilijk. = This lesson is difficult.

So in Dit is de derde les, dit is correct because it is not directly attached to a noun.

In Dutch, nouns take either de or het as their definite article.

• de les = the lesson
• not het les

Since les is a de-word, you say:

• de derde les = the third lesson

The ordinal derde does not change the article. The noun still keeps its normal article.

Because Dutch, like English, uses a special form for ordinal numbers.

• drie = three
• derde = third

Compare:

• Dit is les drie. = This is lesson three.
• Dit is de derde les. = This is the third lesson.

Both can be correct, but they are structured differently. In your sentence, Dutch uses the ordinal form derde.

Derde is the ordinal form of drie.

Dutch ordinals are often made with -de or -ste, but some common ones are irregular:

• een → eerste = first
• twee → tweede = second
• drie → derde = third

So derde is one of the basic ordinal forms you should simply memorize.

This is normal Dutch main-clause word order:

• Dit = subject
• is = verb
• de derde les = predicate noun phrase

Dutch main clauses usually place the finite verb in the second position:

• Dit is de derde les.
• Dat is mijn boek.
• Jan komt morgen.

So the structure is very similar to English here: This is the third lesson.

Because Dutch normally needs the definite article when you mean the third lesson.

• de derde les = the third lesson
• een derde les = a third lesson

Without the article, the phrase sounds incomplete in standard Dutch.

In this sentence, derde already has the form you normally use before a noun.

• de derde les
• het derde boek

So with both de-words and het-words, you usually get derde in this kind of phrase.

You may also see ordinals used on their own:

• de derde = the third one / the third person

But in your sentence, derde is simply the adjective-like ordinal before les.

• dit = this
• dat = that

So:

• Dit is de derde les. = This is the third lesson.
• Dat is de derde les. = That is the third lesson.

In practice, Dutch often uses dit for something close, present, or being introduced, and dat for something farther away or already referred to.

A rough pronunciation guide is:

• Dit ≈ dit
• is ≈ iss
• de ≈ duh or a very short the
• derde ≈ DAIR-duh or DEHR-duh
• les ≈ less

A more Dutch-like pronunciation would be approximately:

dit is də DER-də les

A few notes:

• The r in derde can sound different depending on region.
• The de article is usually unstressed and short.
• les sounds very much like English less, but with a clearer short e.

It is singular.

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

So:

• Dit is de derde les. = singular
• Dit zijn de derde lessen. would not normally be the natural equivalent, because third lessons is a different idea

Yes. That is also possible, especially when numbering lessons, chapters, buses, rooms, and similar items.

Compare:

• Dit is de derde les. = This is the third lesson.
• Dit is les drie. = This is lesson three.

The first uses an ordinal (third), while the second uses the noun plus a cardinal number (lesson three). Both are natural, but they are not identical in style.

Because the subject dit is treated as singular.

• dit is = this is
• deze zijn would be plural only in a different structure

Compare:

• Dit is de derde les. = singular
• Dit zijn de eerste lessen. = These are the first lessons

So is is simply the correct singular form of zijn in the present tense.

No. Only the first word is capitalized because it starts the sentence:

• Dit is de derde les.

Dutch does not capitalize ordinary nouns like English sometimes does in titles, and unlike German, Dutch does not capitalize all nouns. So les stays lowercase.