Learn Hardcore Swedish: Det är i den bokhyllan som dagboken och alla hemliga brev brukar bli gömda. - It is in that bookshelf that the diary and all the secret letters usually get hidden.

Neutral word order (no special emphasis) would be something like:
Dagboken och alla hemliga brev brukar bli gömda i den bokhyllan.
By saying:
Det är i den bokhyllan som dagboken och alla hemliga brev brukar bli gömda,
you put special emphasis on i den bokhyllan – “it is in that bookcase (and not somewhere else)…”

This pattern is:
Det är + [thing you want to emphasize] + som + [rest of the sentence].

So det here is mostly a dummy subject used to build this emphasis structure, not a concrete “it” referring to something specific.

Why do we say i den bokhyllan and not just i bokhyllan?

Den bokhyllan refers to a specific, known bookcase – something both speaker and listener can identify in the context (for example, that bookcase over there).

i bokhyllan = “in the bookcase” (definite, but more neutral; could be “the one we’ve been talking about” without extra emphasis)
i den bokhyllan = “in that (particular) bookcase”

The pattern den + [definite noun] is often used for extra specificity or contrast, similar to English “that …”. So the Swedish here matches the English feeling:
It is in that bookcase that…

What is the difference between den bokhyllan and den där bokhyllan?

Both refer to a specific bookcase, but the nuance is slightly different:

den bokhyllan = “that bookcase (we both know which one)”, often enough when context makes it clear.
den där bokhyllan = “that bookcase over there / that one (over there)”, a bit more demonstrative and often pointing at something physically.

In this sentence, den bokhyllan is already clearly specific; adding där would just make it more explicitly demonstrative:
Det är i den där bokhyllan som…
would also be correct, but slightly more “pointing”.

Why do we need som in Det är i den bokhyllan som dagboken…? Could we leave it out?

In this type of cleft sentence, som is necessary. It introduces the relative/cleft clause that contains the rest of the information.

The pattern is:

Det är X som Y
= “It is X that Y”

Without som, the sentence would be ungrammatical in Swedish in this structure. You cannot say:
✗ Det är i den bokhyllan dagboken och alla hemliga brev brukar bli gömda.

Why is it dagboken (definite) and not just en dagbok or dagbok?

Dagboken is the definite form: “the diary”. The sentence talks about a specific diary that the speaker assumes the listener knows about (or that is uniquely identifiable from context).

en dagbok = “a diary” (indefinite, any diary)
dagbok (bare form) is mostly used for general or abstract statements (e.g. att skriva dagbok = “to keep a diary / journal”).

Here, like in English, it’s not just any diary that gets hidden, but the diary, so Swedish uses dagboken.

Why do we say alla hemliga brev and not alla de hemliga breven?

Both are possible, but the nuance is slightly different:

alla hemliga brev = “all (secret) letters” – more like “every secret letter”, indefinite plural, but still referring to a known set from context.
alla de hemliga breven = “all the secret letters” – more explicitly definite and specific, like a clearly delimited group you’ve already identified.

In many contexts, especially narrative ones, alla hemliga brev feels more natural and fluid in Swedish. It sounds a bit less heavy and still clearly means all of the relevant letters that fit the description.

What does brukar mean here, and how is it different from just using the present tense?

Brukar means “usually / tend(s) to / is (are) in the habit of” and is followed by an infinitive.

brukar bli gömda = “are usually hidden”, “tend to be hidden”.

Compare:

Dagboken och alla hemliga brev blir gömda i den bokhyllan.
= They are hidden in that bookcase (a simple present, could sound like a general fact or current practice).
Dagboken och alla hemliga brev brukar bli gömda i den bokhyllan.
= It’s their habitual or regular fate to be hidden there; it happens typically or normally.

So brukar adds a habitual / usual meaning that simple present doesn’t always clearly give.

Why is it brukar bli gömda and not something like brukar gömmas?

Both are possible Swedish passives, but they feel different:

brukar bli gömda
- Uses bli + past participle (gömda).
- Often emphasizes the event / process of them being hidden repeatedly.
brukar gömmas
- Uses the -s passive (gömmas).
- Grammatically correct, but can feel a bit more formal, static, or bookish in some contexts.

In everyday modern Swedish, bli + participle is very common and sounds natural, especially with brukar to describe repeated events:
De brukar bli gömda = “They usually get hidden.”

Why is it bli gömda and not bli gömt?

The past participle gömd/gömt/gömda agrees in gender and number with the subject when it’s used like this:

dagboken = singular, common gender → gömd
brevet = singular, neuter → gömt
dagboken och alla hemliga brev together form a plural subject → gömda

So with a plural subject (dagboken och alla hemliga brev), you have to use the plural form gömda:
brukar bli gömda = “are usually hidden” (they, plural, are hidden).

Why is the word order som dagboken och alla hemliga brev brukar bli gömda and not som brukar dagboken…?

In a clause introduced by som like this, the word order is normally Subject – Verb – (other stuff), not the V2 (verb-second) order you often see in main clauses.

So:

som dagboken och alla hemliga brev brukar bli gömda
Subject = dagboken och alla hemliga brev
Verb = brukar

Putting the verb second (as in a main clause) would be incorrect here:
✗ som brukar dagboken och alla hemliga brev bli gömda

So Swedish distinguishes between main clause word order (verb-second) and subordinate clause word order (subject–verb), and this is a subordinate clause introduced by som.

What is the difference between blir gömda and är gömda here?

Both are passive-like forms, but they express different aspects:

blir gömda (with bli) often has a dynamic / event feel:
- “get hidden”, “are being hidden (repeatedly)”
- focuses on the action happening.
är gömda (with vara) is more stative:
- “are hidden” in the sense of being in a hidden state.

With brukar, blir gömda works better because you’re talking about what usually happens (a repeated action):
… brukar bli gömda. = They usually get hidden there.

… brukar vara gömda. would lean more toward “are usually in a hidden state there”, which is rarer and sounds a bit odd in many contexts.

Could we say the same thing without the cleft structure, and if so, how would it change the emphasis?

Yes, the neutral version would be:
Dagboken och alla hemliga brev brukar bli gömda i den bokhyllan.

This states the fact without special emphasis on where.
The cleft version:
Det är i den bokhyllan som dagboken och alla hemliga brev brukar bli gömda
strongly emphasizes the location – that specific bookcase – as the key piece of information.

So the meaning is essentially the same, but the cleft structure is used to highlight or contrast the place (e.g. “not in the desk, not under the bed – in that bookcase”).