Learn Hardcore Dutch: Sommige mensen twijfelen aan de waarheid van dat verhaal. - Some people doubt the truth of that story.

twijfelen aan X = to doubt X itself, to question whether X is true, real, reliable.
- Sommige mensen twijfelen aan de waarheid van dat verhaal.
  → They doubt that the story is actually true.
twijfelen over X = to be uncertain how to act, what to decide, or what opinion to have about X.
- Ik twijfel over dat verhaal.
  → I am unsure what to think of the story / whether to believe it or not (more about my decision or stance).

In this sentence the focus is on questioning the truth of the story, so twijfelen aan de waarheid is the natural choice.

Why is it aan de waarheid and not just aan dat verhaal?

You could say:

Sommige mensen twijfelen aan dat verhaal.

This is also correct and idiomatic. It means people doubt the story, usually understood as doubting its truth or credibility.

However, aan de waarheid van dat verhaal is more precise: it explicitly says they doubt the truth of the story, not just the story in some general sense. It slightly emphasizes the truthfulness aspect.

Why is it dat verhaal and not die verhaal?

In Dutch, dat and die both mean that, but:

dat is used with het-words (neuter nouns)
die is used with de-words (common gender nouns) and all plural nouns

The word verhaal (story) is an het-word: het verhaal.
Therefore:

het verhaal → dat verhaal
If it were a de-word, it would be die (for example: de man → die man).

Why is it de waarheid and not het waarheid?

Dutch nouns are either de-words or het-words, and you simply have to learn which each noun is.

waarheid is a de-word: de waarheid
verhaal is a het-word: het verhaal

There is no logical rule here; it is just grammatical gender. So:

de waarheid
de waarheid van dat verhaal

Why is there no article before mensen? Why not de mensen or sommige de mensen?

In Dutch, just like in English, you normally do not use an article with an indefinite plural:

English: Some people doubt the truth… (not the some people)
Dutch: Sommige mensen twijfelen… (not de sommige mensen)

If you said de mensen, it would mean the people in a specific group:

Sommige van de mensen twijfelen aan de waarheid van dat verhaal.
→ Some of the people (in a previously mentioned group) doubt the truth of that story.

But in the original sentence we are talking about people in general, so we just say sommige mensen.

Could we say sommigen instead of sommige mensen?

Yes, but only in a slightly different construction:

Sommige mensen twijfelen aan de waarheid van dat verhaal.
→ Some people doubt the truth of that story.
Sommigen twijfelen aan de waarheid van dat verhaal.
→ Some (of them) doubt the truth of that story.

sommigen is a pronoun (like English some) and usually refers back to a group that is clear from context:

Veel mensen geloven het verhaal, maar sommigen twijfelen aan de waarheid ervan.
→ Many people believe the story, but some doubt the truth of it.

So sommigen is fine when the group is known; sommige mensen explicitly names the group (mensen).

Why is the verb twijfelen and not twijfelt?

The subject is sommige mensen, which is plural.
In the present tense:

ik twijfel
jij twijfelt
hij/zij/het twijfelt
wij twijfelen
jullie twijfelen
zij twijfelen

With plural subjects (wij/jullie/zij, or any plural noun like mensen), you use twijfelen.

So:

De man twijfelt aan de waarheid. (singular)
Sommige mensen twijfelen aan de waarheid. (plural)

Could we move aan to the end, like Sommige mensen twijfelen de waarheid van dat verhaal aan?

No. That kind of verb–particle split works with separable verbs (like opbellen, aankomen), but twijfelen aan is not a separable verb. It is just a verb plus a preposition.

The preposition aan must stay with its object:

Correct: Sommige mensen twijfelen aan de waarheid van dat verhaal.
Incorrect: ✗ Sommige mensen twijfelen de waarheid van dat verhaal aan.

You can move the whole phrase in some word orders (for example in a subordinate clause):

…omdat sommige mensen aan de waarheid van dat verhaal twijfelen.

Why is it de waarheid van dat verhaal and not de waarheid over dat verhaal?

Here van expresses an of relationship, like a possession or belonging:

de waarheid van dat verhaal = the truth of that story

If you say de waarheid over dat verhaal, it sounds more like:

the truth about that story
the true information concerning that story

That is possible in some contexts, but the fixed expression when you doubt whether something is true is normally aan de waarheid van X twijfelen.

Is there any difference between sommige mensen and enkele mensen here?

Both are often translated as some people, but there is a nuance:

sommige mensen
- neutral some people
- often just indicates a subset, without much implication about size.
enkele mensen
- typically suggests a few people, often a small number.
- can sound slightly more limited than sommige.

In this sentence you could say:

Enkele mensen twijfelen aan de waarheid van dat verhaal.
→ A few people doubt the truth of that story.

This suggests it is not many. Sommige mensen does not automatically suggest small or large; it is more neutral.

How can I replace aan de waarheid van dat verhaal with a pronoun?

The pronoun that replaces aan + something is usually daaraan (or eraan in spoken language):

Sommige mensen twijfelen aan de waarheid van dat verhaal.
Sommige mensen twijfelen daaraan.

If the thing you refer to is mentioned right before, daaraan is very natural:

Is dat verhaal echt gebeurd? Sommige mensen twijfelen daaraan.
→ Did that story really happen? Some people doubt it.