Learn Hardcore Swedish: Han lämnar aldrig sina viktigaste saker i bilen. - He never leaves his most important things in the car.

hans = his (someone else’s), neutral, not tied to the subject
sin / sitt / sina = his/her/its own, i.e. belongs to the subject of the clause

In Han lämnar aldrig sina viktigaste saker i bilen, the things belong to han (the subject).
So Swedish uses the reflexive possessive: sina.

If you said:

Han lämnar aldrig hans viktigaste saker i bilen, it would usually mean:
He never leaves *another man’s most important things in the car.*

So sina = his own, hans = another man’s (in this context).

What is the difference between sin, sitt, and sina?

All three mean his/her/its/their own, but they agree with the thing owned, not with the owner.

sin – for en-words, singular
- sin bil (his/her own car) – bil is an en-word
sitt – for ett-words, singular
- sitt hus (his/her own house) – hus is an ett-word
sina – for plural (both en- and ett-words)
- sina saker (his/her own things) – saker is plural

In the sentence, saker is plural, so we must use sina.

Why is it viktigaste and not just viktiga or viktig?

Viktigaste is the superlative form of viktig (important):

viktig = important
viktigare = more important (comparative)
viktigast / viktigaste = most important (superlative)

When a superlative adjective is used with a definite form or a possessive (mina, dina, sina, hans, hennes, etc.), it normally appears as viktigaste:

mina viktigaste saker – my most important things
de viktigaste böckerna – the most important books

So sina viktigaste saker = his own most important things.
Viktiga would just mean important, not most important.

Why is saker indefinite (no -na ending) but bilen is definite?

Swedish often uses:

indefinite plural for things spoken about in a general way
definite singular when referring to a specific item that is clear from context

Here:

sina viktigaste saker – his most important things in general, not a specific, known set that has already been identified in the conversation. So saker is indefinite plural.
i bilen – in the car; in context, it’s understood we are talking about his (or their) car in the situation. Swedish uses the definite: bilen.

If you said i en bil, it would sound like “in a car (any car)”, which is less natural for this habitual statement.

Why does aldrig come after lämnar and not before it?

In main clauses, Swedish follows the V2 rule: the finite verb (here lämnar) normally comes in second position.

Word order in the sentence:

Han (subject – 1st position)
lämnar (verb – must be 2nd)
aldrig (adverb)
sina viktigaste saker i bilen (rest of the sentence)

So aldrig has to come after the verb in a simple main clause like this.

This is why:

Han lämnar aldrig ... is correct.
Han aldrig lämnar ... is wrong in a normal main clause.

Could you ever say Han aldrig lämnar sina viktigaste saker i bilen?

Not as a normal main clause. For a neutral statement, it is ungrammatical.

You can move aldrig before the verb in some subordinate clauses or special sentence types, but then the structure around it changes. For example:

Jag vet att han aldrig lämnar sina viktigaste saker i bilen.
I know that he never leaves his most important things in the car.

Here att (that) takes the first position in the subordinate clause, so han no longer has to be first, and the V2 rule doesn’t apply in the same way. But in a standalone main clause, it must be Han lämnar aldrig ...

Why is the tense lämnar (present) used to mean a general habit?

Swedish present tense is used for:

actions happening right now
general truths and habits
sometimes future events (especially when clearly future from context)

Here it expresses a habitual action:

Han lämnar aldrig sina viktigaste saker i bilen.
= He never (as a rule, habitually) leaves his most important things in the car.

This is the same as in English with the simple present:
He never leaves his most important things in the car.

Is there a difference between lämnar and lämnar kvar?

Yes, there is a small nuance:

lämna on its own can mean leave (not take with you) or leave (abandon, hand over, etc.), depending on context.
lämna kvar focuses more on the idea of leaving something behind, often where it already is.

In this sentence:

Han lämnar aldrig sina viktigaste saker i bilen.
Already clearly means he doesn’t leave them in the car (as in English).

You could say lämnar kvar i bilen, but it would be more explicit that he leaves them behind in the car. The simple lämnar is completely natural and sufficient here.

Why is it i bilen and not something like på bilen?

The preposition i means in / inside, and that’s the usual one for containers, rooms, vehicles, etc.:

i bilen – in the car
i huset – in the house
i väskan – in the bag

På means on / on top of or sometimes at, depending on context:

på bilen – on the car (on the roof, hood, etc.)
på bordet – on the table

Since the meaning is inside the car, i bilen is the correct choice.

Could you say i sin bil instead of i bilen?

Yes, you could say:

Han lämnar aldrig sina viktigaste saker i sin bil.

This emphasizes that it is his own car (reflexive possessive sin + bil).
However, i bilen often already implies his (the relevant) car from context, so i bilen is more neutral and very common.

Both are grammatically correct; the choice is about nuance and context.

What are the genders and basic forms of sak and bil?

Both are en-words (common gender):

en sak – a thing
- plural indefinite: saker
- plural definite: sakerna
en bil – a car
- definite singular: bilen (the car)
- plural indefinite: bilar
- plural definite: bilarna

In the sentence:

sina viktigaste saker – indefinite plural
i bilen – definite singular

Why aldrig and not inte in this sentence?

Alldrig and inte have different meanings:

inte = not (negation)
- Han lämnar inte sina saker i bilen.
  = He does not leave his things in the car. (Maybe sometimes, maybe not; context-dependent.)
aldrig = never (not at any time)
- Han lämnar aldrig sina viktigaste saker i bilen.
  = He never leaves his most important things in the car. (At no time, as a rule.)

So aldrig expresses a stronger, absolute negation over time.

Can the word order be changed, like I bilen lämnar han aldrig sina viktigaste saker?

Yes, you can move i bilen to the front for emphasis:

I bilen lämnar han aldrig sina viktigaste saker.

This is still grammatical because Swedish keeps the V2 rule:

I bilen = first element (not the subject this time)
lämnar = verb in second position
han = subject
aldrig sina viktigaste saker = rest

The meaning is almost the same, but i bilen is now in a more emphatic or contrastive position, as if you’re stressing in the car, specifically, he never leaves them.