Learn Hardcore Swedish: Hon är ordentlig i köket, men hennes bror är ganska slarvig när han lagar mat. - She is tidy in the kitchen, but her brother is quite careless when he cooks.

ordentlig means neat, tidy, orderly, careful.
- Hon är ordentlig i köket = She is tidy/careful in the kitchen.
slarvig means sloppy, careless, untidy.
- han är ganska slarvig när han lagar mat = he is quite careless when he cooks.

Both are very common everyday words, especially to describe how someone behaves, works, or keeps their things.

What is the nuance of ganska in ganska slarvig? Is it “quite” or “rather” or “pretty”?

Ganska usually corresponds to English quite / rather / pretty (in the sense of pretty good / pretty messy).

ganska slarvig = quite sloppy / rather careless / pretty sloppy

It softens the adjective a bit. Stronger options would be väldigt slarvig (very sloppy) or otroligt slarvig (incredibly sloppy).
Weaker: lite slarvig (a bit sloppy).

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

In Swedish, you are typically in a room:

i köket = in the kitchen
i vardagsrummet = in the living room
i badrummet = in the bathroom

På is used with some places (e.g. på jobbet = at work, på kontoret = at the office), but for standard rooms in a home, i is the normal preposition.

Why is it köket and not just kök?

Kök is kitchen (indefinite), and köket is the kitchen (definite).

In Swedish, when you talk about a specific, known place like the kitchen in the home, you almost always use the definite form:

i köket = in the kitchen (in the house)
i ett kök = in a kitchen (some kitchen, not specific; less common in this kind of sentence)

Why is it hennes bror and not sin bror?

Hennes means her (possessive pronoun).
Sin/sitt/sina is a reflexive possessive: it refers back to the subject of the same clause.

In the sentence:

First clause: Hon är ordentlig i köket
Second clause: men hennes bror är ganska slarvig…

The subject of the second clause is hennes bror (her brother).
If you said men sin bror är ganska slarvig, sin would refer to its own subject, which would be bror—that doesn’t make sense (“but its own brother is quite sloppy”).

So you must use hennes bror to say her brother.

Could you say Hon är ordentlig i köket, men hennes bror är ganska slarvig i köket instead?

Yes, grammatically it’s fine:

Hon är ordentlig i köket, men hennes bror är ganska slarvig i köket.

That means: She is tidy in the kitchen, but her brother is quite sloppy in the kitchen.

The original sentence avoids repetition by replacing the second i köket with när han lagar mat (when he cooks), which sounds a bit more natural and informative.

What does när han lagar mat literally mean, and why is it present tense?

Literally:

när = when
han = he
lagar = cooks / prepares
mat = food

So när han lagar mat = when he cooks (food).

The present tense lagar is used for general, repeated situations or habits, just like English:

Han är slarvig när han lagar mat.
= He is sloppy when he cooks (in general / habitually).

Why is it lagar mat and not just lagar or something like gör mat?

The normal way to say cook (food) is laga mat:

Jag lagar mat. = I’m cooking.
Han gillar att laga mat. = He likes to cook.

Laga alone usually means repair / fix (e.g. laga bilen = fix the car).
Göra mat is understandable but not idiomatic; laga mat is the standard expression.

Why is it lagar mat and not lagar maten?

Mat is a mass noun (like food in English). In general statements about cooking, Swedish uses mat without the definite ending:

Han lagar mat. = He cooks / he is cooking (food).
Hon lagar ofta mat. = She often cooks.

Lagar maten (cooks the food) would suggest some specific food already known from the context, and it would sound a bit strange in this generic description of his habits.

Why is it är ordentlig and är slarvig, and not something like är ordentligt or slarviga?

Adjectives in Swedish agree with the noun they describe:

common gender (en-word) singular: basic form
- en ordentlig person
- en slarvig kille
neuter (ett-word) singular: -t
- ett ordentligt barn
plural: -a
- ordentliga barn
- slarviga personer

Here, the adjectives describe hon (she) and hennes bror (her brother), both understood as en-personer (common gender), singular. So you use the basic forms:

Hon är ordentlig.
Hennes bror är slarvig.

That’s why it’s not ordentligt or slarviga here.

Can you change the word order, like Hon är i köket ordentlig?

No, that sounds wrong in Swedish. The natural order is:

Hon är ordentlig i köket.

Adjective + place normally follows this pattern:

Han är duktig i skolan. (He is good at school.)
Hon är stressad på jobbet. (She is stressed at work.)

Putting i köket between är and ordentlig is not idiomatic in this kind of sentence.

What does men mean here, and can it start a sentence?

Men means but:

… i köket, men hennes bror …
= … in the kitchen, but her brother …

Yes, men can also start a sentence:

Hon är ordentlig i köket. Men hennes bror är ganska slarvig när han lagar mat.

That’s perfectly fine in Swedish, especially in spoken language and informal writing.