De les duurt dertig minuten.

Because les is a de-word in Dutch. Nouns in Dutch take either de or het, and this is something you usually have to learn together with the noun.

So:

• de les = the lesson / the class

There is no separate rule from the sentence itself that tells you this; it is just part of the noun.

It can mean either, depending on context. In this sentence, les can refer to:

• a lesson
• a class period
• sometimes even a teaching session more generally

So an English speaker might translate it as The lesson lasts thirty minutes or The class lasts thirty minutes, depending on the situation.

The infinitive is duren, which means to last.

Here, the subject is de les, which is singular, so Dutch uses the third-person singular form:

• duren → duurt

Comparable pattern:

• ik duur
• jij duurt
• de les duurt
• wij duren

So duurt is simply the correct present-tense form for a singular subject like de les.

Because Dutch normally uses duren to express duration.

So Dutch prefers:

• De les duurt dertig minuten.

Literally, this is The lesson lasts thirty minutes.

English often uses be in some time expressions, but in Dutch, for duration, duren is very natural and common.

In Dutch, you usually do not need a preposition here. The duration can come directly after duren:

• De les duurt dertig minuten.

So Dutch structure is closer to:

• The lesson lasts thirty minutes

Even though English often says for thirty minutes, Dutch leaves that idea unexpressed because duren already makes the meaning clear.

Because dertig means thirty, and anything above one normally takes the plural noun in Dutch.

So:

• één minuut = one minute
• twee minuten = two minutes
• dertig minuten = thirty minutes

That is very similar to English:

• one minute
• thirty minutes

Yes. This is a straightforward Dutch main clause:

• De les = subject
• duurt = verb
• dertig minuten = duration expression

So the order is:

subject + verb + rest

That is the normal pattern for a simple statement in Dutch.

Yes, absolutely. That is also correct Dutch.

Compare:

• De les duurt dertig minuten.
• De les is dertig minuten lang.

Both mean basically the same thing.

A small difference:

• duurt focuses on the verb to last
• is ... lang describes the length/duration more like a property

In everyday Dutch, both are natural.

No. With duren, lang is not needed.

Correct:

• De les duurt dertig minuten.

Also correct, but different structure:

• De les is dertig minuten lang.

But you would not normally combine them as:

• De les duurt dertig minuten lang.

That sounds unnecessary, because duurt already expresses duration.

The past tense of duren is usually duurde for singular subjects.

So:

• De les duurde dertig minuten. = The lesson lasted thirty minutes.

And in the present perfect:

• De les heeft dertig minuten geduurd. = The lesson has lasted / lasted thirty minutes.

This is useful because Dutch learners often want to reuse the same sentence in different tenses.