Duizend mensen wachten.

Because the subject is plural: duizend mensen.

In the present tense, Dutch verbs usually work like this:

• ik wacht
• jij/u wacht
• hij/zij wacht
• wij/jullie/zij wachten

Since mensen is plural, the verb takes the plural form wachten.

In normal Dutch, duizend by itself means one thousand. So:

• duizend mensen = one thousand people

You usually do not say een duizend mensen in standard everyday Dutch.

Compare:

• honderd mensen = a hundred people
• duizend mensen = a thousand people

Because Dutch, like English, often uses no article with indefinite plural nouns.

So:

• mensen = people, some people
• de mensen = the people

In Duizend mensen wachten, the phrase is indefinite: it means a certain number of people, not a specific known group.

The singular is mens.

• een mens = one person / a human being
• mensen = people

A learner should know that Dutch often uses persoon/personen too, especially in more formal contexts:

• duizend mensen sounds natural in everyday language
• duizend personen sounds more formal or official

Yes. This is a very basic Dutch main-clause pattern:

• subject
  • verb

So:

• Duizend mensen = subject
• wachten = verb

This is the same kind of order as:

• De kinderen spelen
• Veel studenten werken
• Duizend mensen wachten

Yes, absolutely.

Both are correct:

• Duizend mensen wachten
• Er wachten duizend mensen

The version with er is very common when you are introducing the existence of something, similar to English There are...

So:

• Er wachten duizend mensen = There are a thousand people waiting

Meanwhile, Duizend mensen wachten puts duizend mensen more directly in the subject position.

A rough English-friendly guide:

• duizend ≈ DOW-zent
  
  • ui is a Dutch sound that does not exist exactly in English, so this is only approximate.
• mensen ≈ MEN-suh(n)
• wachten ≈ WAKH-tuh(n)
  
  • The ch is the throaty Dutch sound, like in Scottish loch.

A few extra notes:

• The final -en in speech is often weakened, especially in casual Dutch.
• g and ch can sound quite strong depending on the region.

It is capitalized because it is the first word of the sentence.

The word itself is not normally a proper noun, so in the middle of a sentence it would be lowercase:

• Er wachten duizend mensen.

Not exactly. It is a cardinal number. It comes before the noun and tells you how many there are.

So in:

• duizend mensen

duizend functions as a number word modifying mensen.

It does not change form here. Dutch number words are generally simple in this position:

• drie mensen
• honderd mensen
• duizend mensen

Yes. In simple present Dutch, wachten can often correspond to either English wait or are waiting, depending on context.

So this sentence could match English ideas like:

• A thousand people wait
• A thousand people are waiting

Dutch uses the simple present more broadly than English sometimes does.

Normally, when you are counting individual people or things, the noun is plural:

• duizend mensen
• duizend huizen
• duizend boeken

So in this sentence, mensen must be plural.

There are some special expressions in Dutch where a number is followed by a singular-looking noun in fixed or collective usage, but for ordinary counting, use the plural noun after duizend.

No, not exactly.

• Duizend mensen = exactly one thousand people
• Duizenden mensen = thousands of people

So duizenden is less exact and means a large number, more than one thousand in a general sense.

Sometimes, in informal Dutch, you may hear man used after a number to mean people:

• duizend man

But this is more colloquial and somewhat different in tone. For learners, duizend mensen is the safest and most neutral choice.

So:

• duizend mensen wachten = standard, neutral
• duizend man wachten = possible in informal usage