Learn Hardcore Swedish: Det är hennes största hemlighet att ingen annan får läsa dagboken. - It is her biggest secret that no one else is allowed to read the diary.

Starting with Det är is very common in Swedish. Here det is a dummy subject (like English “it” in “It is important that...”). This structure:

Det är + [something] + att ...

is used to introduce and emphasize what comes after, in a slightly more neutral or “framed” way.

The version without det:

Hennes största hemlighet är att ...

sounds a bit more direct and focused on hennes största hemlighet.

So:

Det är ... = a common, slightly more neutral way to present the information.
Hennes största hemlighet är ... = puts the phrase hennes största hemlighet more in the spotlight.

In everyday Swedish, you’ll hear both.

Why is it hennes and not sin here?

Swedish has two types of possessives:

hennes / hans / deras = non‑reflexive (“her / his / their”)
sin / sitt / sina = reflexive (“her own / his own / their own”), referring back to the subject of the clause.

In the sentence:

Det är hennes största hemlighet att ingen annan får läsa dagboken.

the grammatical subject is det, not the woman whose secret it is. Because the person is not the subject of the clause, you cannot use sin here. Reflexive sin/sitt/sina must refer to the subject of that clause, and here the subject is just dummy det.

So:

Det är hennes största hemlighet ... ✅ (correct)
× Det är sin största hemlighet ... ❌ (wrong)

To use sin, the person you refer to has to be the subject of that clause, for example:

Hon har en hemlighet. Det är hennes största hemlighet att ingen annan får läsa hennes dagbok.
Hon läser sin dagbok. = She is reading her own diary.
Hon läser hennes dagbok. = She is reading some other woman’s diary.

In your sentence, hennes is the only grammatical choice.

Why is it största and not störst?

Swedish superlatives usually have two forms:

störst – used predicatively (after är, blir, etc.):
- Hemligheten är störst. = “The secret is biggest.”
största – used attributively, before a noun in a definite/possessive phrase:
- den största hemligheten
- hennes största hemlighet

In your sentence, the adjective comes before the noun and is part of a phrase that is definite in meaning (because of hennes):

hennes största hemlighet

So we must use the -a form största, not störst.

Compare:

Den här hemligheten är störst. (predicative)
Det här är hennes största hemlighet. (attributive)

Why is it hennes största hemlighet and not hennes största hemligheten?

In Swedish, when you use a possessive (like min, din, hans, hennes, vår, er, deras), you:

do not use a definite article (den, det, de)
do not put the -en / -et / -na definite ending on the noun.

So you say:

hennes hemlighet = her secret
hennes största hemlighet = her biggest secret

but not:

× hennes hemligheten
× hennes största hemligheten

Even though in English we say “her biggest secret”, which feels very definite, Swedish marks the definiteness only through the possessive (hennes). The noun hemlighet stays in the indefinite form.

So the correct structure is:

[possessive] + [adjective] + [indefinite noun]
hennes största hemlighet

What grammatical role does att ingen annan får läsa dagboken have in this sentence?

The clause:

att ingen annan får läsa dagboken

is an att-clause (a subordinate clause introduced by att) and it functions as a predicative complement (also called a content clause or complement clause).

The overall structure is:

Det – dummy subject
är – verb
hennes största hemlighet – subject complement (what “it” is)
att ingen annan får läsa dagboken – further complement explaining what that secret is

So you can think of it like:

It is her biggest secret [that no one else is allowed to read the diary].

That bracketed part is exactly what the att-clause is doing in Swedish.

Is att necessary here? Could I say Det är hennes största hemlighet ingen annan får läsa dagboken?

In standard written Swedish, att is necessary in this kind of clause.

The version without att:

× Det är hennes största hemlighet ingen annan får läsa dagboken.

sounds ungrammatical or at least very colloquial and “broken” to a native speaker.

Here att is a subordinating conjunction, introducing the content of the secret:

att ingen annan får läsa dagboken
= that no one else may read the diary.

In casual spoken Swedish, people do sometimes drop att, especially before verbs like ska, kan, måste, etc.:

Jag tror (att) han kommer.

But in sentences like yours, where att introduces the “content” of the secret, it is not normally omitted in standard language. So you should keep it:

Det är hennes största hemlighet att ingen annan får läsa dagboken. ✅

Why is it ingen annan får and not ingen annan kan or ska?

The verb få (in får) very often expresses permission or being allowed to:

Får jag gå nu? = Am I allowed to go now?
Du får inte röka här. = You are not allowed to smoke here.

In your sentence:

ingen annan får läsa dagboken

means:

“no one else is allowed to read the diary.”

If you used kan instead:

ingen annan kan läsa dagboken
= “no one else can read the diary” (maybe they are unable to, can’t read the handwriting, don’t know the language, etc.)

If you used ska:

ingen annan ska läsa dagboken
= “no one else shall / is supposed to read the diary” (more about a rule or plan, less about simple permission).

The idea here is a prohibition based on permission, so får is the most natural choice.

Why is the word order att ingen annan får läsa dagboken and not att får ingen annan läsa dagboken?

Swedish has different word-order rules for main clauses and subordinate clauses.

Main clause (independent sentence):
The verb usually goes in second position (V2 rule):
- Ingen annan får läsa dagboken.
  (Subject ingen annan in first position, verb får in second.)
Subordinate clause introduced by att:
The word order is typically:
- att + subject + finite verb + ...
So you get:
- att ingen annan får läsa dagboken
  (att – subordinating conjunction
  ingen annan – subject
  får – finite verb)

The version:

× att får ingen annan läsa dagboken

uses main‑clause (V2) order inside a subordinate clause, which is wrong in standard Swedish. So you must keep:

att ingen annan får läsa dagboken. ✅

Why is it dagboken and not dagbok, en dagbok, or hennes dagbok?

dagboken is the definite form:

dagbok = diary (indefinite)
en dagbok = a diary
dagboken = the diary

In the sentence:

att ingen annan får läsa dagboken

we are clearly talking about a specific diary that is already known from context (almost certainly her diary). In Swedish it is very common to use the simple definite form in such cases.

You could also say:

att ingen annan får läsa hennes dagbok.

This is also correct and explicitly says her diary. The difference is:

dagboken – “the diary” (understood from context whose it is)
hennes dagbok – “her diary” (explicitly marking whose diary)

What you cannot say in standard Swedish is:

× hennes dagboken

because with a possessive (hennes) you cannot add the definite ending -en to the noun.

So the given form dagboken is natural, and hennes dagbok would also work but with a slightly stronger emphasis on whose diary it is.

Could you also say läsa i dagboken instead of läsa dagboken? Is there a difference?

Yes, you can say both:

läsa dagboken
läsa i dagboken

Both can mean “read the diary,” but there is a small nuance:

läsa dagboken
often feels a bit more like reading the diary as a whole text (or at least reading it in a more complete/intentional way).
läsa i dagboken
literally “read in the diary” – can suggest browsing, reading parts, flipping through it. For many speakers, though, this nuance is quite weak, and both are used very similarly.

In your sentence, both are acceptable:

att ingen annan får läsa dagboken ✅
att ingen annan får läsa i dagboken ✅

The prohibition is the same: no one else is allowed to read her diary.