Learn Hardcore Danish: Det er ikke nødvendigt, at han bekymrer sig så meget om små fejl. - It is not necessary that he worries so much about small mistakes.

That version is grammatically possible but sounds very heavy and unnatural. Danish, like English, almost always uses det to “carry” the verb and then puts the content clause later:

Det er ikke nødvendigt, at han bekymrer sig så meget om små fejl.

So:

You cannot normally drop det here.
You normally say Det er ikke nødvendigt, at …, not Er ikke nødvendigt, at …

Why is it at han bekymrer sig and not something like at han sig bekymrer?

The word order in a finite subordinate clause (a clause introduced by at, fordi, når, etc.) is:

Subject – (adverbs) – verb – (objects / reflexives / other stuff)

So we get:

han (subject)
bekymrer (finite verb)
sig (reflexive pronoun)

→ at han bekymrer sig …

You never split verb + reflexive like han sig bekymrer. That word order doesn’t exist in Danish.

Why do we need sig after bekymrer? Could it just be bekymrer?

Danish distinguishes between:

bekymre (non‑reflexive, transitive)
- to worry someone / to cause worry
- Example: Det bekymrer mig, at han arbejder så meget.
  It worries me that he works so much.
bekymre sig (om / for / over) (reflexive)
- to worry (oneself), to be worried (about)
- Example: Han bekymrer sig om små fejl.
  He worries about small mistakes.

In your sentence we mean that he is worrying, not that he is worrying someone else. Therefore you must use the reflexive:

han bekymrer sig … = he worries (himself)

Without sig, it would mean something like:

it is not necessary that he worries [someone] so much about small mistakes

—which would be a different meaning and also incomplete without an object (who he worries).

Why is it om små fejl and not for små fejl or over små fejl?

The usual preposition with bekymre sig for “worry about something” (things, issues, errors) is om:

bekymre sig om noget = to worry about something

So:

Han bekymrer sig om små fejl.
He worries about small mistakes.

Other combinations exist, but they have nuances:

bekymre sig for nogen/noget
Often used when you are worried for someone’s safety, health, future, etc.:
- Hun bekymrer sig for sine børn.
  She worries about her children.
bekymre sig over noget
Can be used, but sounds more like being upset/concerned over something that has happened.

For “worrying about small mistakes”, om is the most natural choice.

What does så meget add here? Could we say for meget instead?

så meget = so much, so greatly
It describes a high degree, without directly judging it as “too much.”
- Han bekymrer sig så meget.
  He worries so much.
for meget = too much
This explicitly says that the amount is excessive.
- Han bekymrer sig for meget.
  He worries too much.

In your sentence:

Det er ikke nødvendigt, at han bekymrer sig så meget om små fejl.

This corresponds well to English “He doesn’t need to worry so much about small mistakes” — it implies “too much,” but in a slightly softer, more descriptive way. You could use for meget:

Det er ikke nødvendigt, at han bekymrer sig for meget om små fejl.

but that sounds more like a direct judgment that his worrying is excessive. Both are grammatical; the nuance changes slightly.

Why is it små fejl and not lille fejl?

Two things are going on:

Number (singular vs plural)
- en lille fejl = a small mistake (one)
- små fejl = small mistakes (plural)
Adjective form
The adjective lille has special forms:
- Singular common gender: lille (en lille fejl)
- Singular neuter: lille (et lille hus)
- Plural (both genders): små (små fejl, små huse)

So when you have fejl in the plural:

små fejl = small mistakes
(literally: small errors)

You could say:

en lille fejl – one small mistake
flere små fejl – several small mistakes

Is the comma before at required? Could it be written without the comma?

Modern Danish allows two comma systems. In practice:

Det er ikke nødvendigt, at han bekymrer sig så meget om små fejl.
Det er ikke nødvendigt at han bekymrer sig så meget om små fejl.

Both can be correct, depending on which system you follow.

However, in real life:

Many Danes do put a comma before at when it introduces a full finite clause (with a subject and conjugated verb), like at han bekymrer sig …
Leaving out the comma is also allowed under the “new comma” rules, but you should be consistent in a longer text.

For a learner, it’s perfectly safe (and common) to include the comma here.

Could we drop han and just say Det er ikke nødvendigt at bekymre sig så meget om små fejl?

Yes, that’s a perfectly natural alternative:

Det er ikke nødvendigt at bekymre sig så meget om små fejl.
It is not necessary to worry so much about small mistakes.

Differences in meaning:

at han bekymrer sig
→ specifically he (a particular person) is worrying.
at bekymre sig (impersonal infinitive)
→ more general: to worry (in general). It can still be understood as “for him” from context, but grammatically it’s impersonal.

So if you want to emphasize that this particular person doesn’t need to worry, keep han:

Det er ikke nødvendigt, at han bekymrer sig så meget …

If you’re talking more generally about the action, use the infinitive version.

Why is it han and not ham after at?

In at han bekymrer sig …, han is the subject of the subordinate clause. In Danish (as in English), the subject pronoun form is used:

han = he (subject form)
ham = him (object form)

So:

Det er ikke nødvendigt, at han bekymrer sig …
It’s not necessary that he worries … ✅

Using ham here would be the object form and grammatically wrong:

✗ at ham bekymrer sig

Why is the negation placed as er ikke nødvendigt and not er nødvendigt ikke?

In main clauses, the usual order around ikke is:

Subject – finite verb – ikke – complements / other stuff

So you get:

Det (subject)
er (finite verb)
ikke (negation)
nødvendigt (adjective complement)

→ Det er ikke nødvendigt …

The pattern er nødvendigt ikke isn’t standard Danish word order. You generally don’t put ikke after a short predicative adjective like that.

Is there another common way to say “it is not necessary that he worries so much about small mistakes” in Danish?

Yes, a very natural alternative is with behøver ikke:

Han behøver ikke at bekymre sig så meget om små fejl.
He doesn’t need to worry so much about small mistakes.

This shifts the focus:

Det er ikke nødvendigt, at han bekymrer sig …
→ “It is not necessary that he worries …” (more formal / impersonal, with det as dummy subject)
Han behøver ikke at bekymre sig …
→ “He doesn’t need to worry …” (more direct about his obligation/need)

Both are fully correct and common.

What exactly does nødvendigt mean here? Is it more like “required,” “needed,” or “necessary”?

nødvendigt is the adjective nødvendig in neuter form, and it generally means:

necessary
needed
required (in some contexts)

In this exact sentence:

Det er ikke nødvendigt, at han bekymrer sig så meget om små fejl.

the most natural English equivalents are:

It is not necessary that he worries so much about small mistakes.
He doesn’t need to worry so much about small mistakes.

“Required” would sound a bit too formal/strong here in English, though it’s not totally wrong in meaning.