Learn Hardcore Finnish: Ostin uuden pölynimurin, koska vanha oli hidas. - I bought a new vacuum cleaner because the old one was slow.

Finnish conjugates the verb for person and number, so the subject is built into the verb. Ostin already means “I bought” (1st person singular -n). You can add Minä ostin for emphasis or contrast, but it’s optional.

Why do uuden and pölynimurin end in -n?

They mark a singular total object (the whole thing was bought). For ordinary nouns, the singular total object takes the genitive form, and adjectives agree with the noun:

uusi → uuden
pölynimuri → pölynimurin

Is that case called genitive or accusative?

Both labels are used. For regular nouns in the singular, the total object looks like the genitive (pölynimurin, uuden). Many books call this the “genitive (of total object)” or “genitive-accusative.” Personal pronouns have a distinct accusative (minut, sinut, etc.).

Why can’t I say Ostin uusi pölynimuri?

Two issues:

Adjectives must agree with the noun in case and number: uusi → uuden.
A singular total object can’t be bare nominative. The correct form is Ostin uuden pölynimurin. (In the plural, total objects are nominative plural: Ostin uudet pölynimurit.)

When would I use the partitive uutta pölynimuria instead?

Use the partitive object when:

The action is negated: En ostanut uutta pölynimuria “I didn’t buy a new vacuum cleaner.”
The action is incomplete/ongoing or attempted: Ostin uutta pölynimuria ≈ “I was (in the process of) buying a new vacuum cleaner.” (Less common with count nouns, but possible with the right context.)
You mean a partial amount (typical with mass nouns, not so much with vacuum cleaners).

Why is there a comma before koska? Can I switch the clause order?

Finnish always uses a comma between a main clause and a subordinate clause. So: Ostin uuden pölynimurin, koska vanha oli hidas. You may also put the reason first: Koska vanha oli hidas, ostin uuden pölynimurin.

How do koska, sillä, kun, and että differ?

koska = because (subordinator). Neutral, the default for giving reasons.
sillä = for/because (coordinating conjunction). More formal; you generally don’t place the sillä-clause first: Ostin …, sillä vanha oli hidas.
kun = when; in colloquial speech it often stands in for “because,” but in standard Finnish use koska for “because.”
että = that (complementizer), not “because,” so it doesn’t fit here.

How can vanha alone mean “the old one”?

Finnish commonly uses adjectives as nouns when the referent is obvious from context. Here vanha means “the old (vacuum cleaner).” You could spell it out: koska vanha imuri oli hidas, or add a pronoun: koska se vanha oli hidas.

Why is it vanha, not vanhan, in koska vanha oli hidas?

Because vanha is the subject of that clause, and subjects are in the nominative. Vanhan is genitive and would be wrong here by itself. You would use genitive if it modifies another noun, e.g., koska vanhan moottori oli rikki “because the old one’s motor was broken.”

Why oli hidas and not oli hidasta?

With olla (“to be”), a predicative adjective is typically in the nominative with a definite, countable subject: (se) oli hidas “it was slow.” The partitive predicative (hidasta) is used with partitive/indefinite subjects or when describing an activity in general, e.g., Se työ oli hidasta “That work was slow.”

How are ostin and oli formed?

ostaa (to buy), type 1: present ostan; past adds -i- plus the personal ending: ostin “I bought.”
olla (to be) is irregular: present 3sg on, past 3sg oli.

What does pölynimuri literally mean, and how does it inflect?

It’s a compound: pöly (dust) + linking n + imuri (sucker) → “dust-sucker.” Compounds inflect at the end:

Nominative: pölynimuri
Genitive: pölynimurin
Partitive: pölynimuria A common shorter word is imuri. The spelling pölyimuri is nonstandard; the correct form has the linking n: pölynimuri.

Can I omit the noun in the first clause too?

Yes. Ostin uuden, koska vanha oli hidas. Both uuden and vanha are understood from context as referring to a vacuum cleaner.

Are there colloquial variants of this sentence?

Yes. Spoken Finnish might be: Mä ostin uuden imurin, ku se vanha oli hidas. Notes:

mä = I (colloquial)
ku often stands for kun, which in speech can mean “because”
Adding se before vanha is very natural in speech.

Where are the English articles “a” and “the”?

Finnish has no articles. Definiteness is read from context, case, word order, or pronouns. Here uuden pölynimurin corresponds to “a new vacuum cleaner,” and vanha is understood as “the old one.”

Why use the adjective hidas (“slow”) and not the adverb hitaasti (“slowly”)?

After olla (“to be”), Finnish uses an adjective as a predicative: (se) oli hidas “it was slow.” Use the adverb for manner with action verbs: Se toimi hitaasti “It operated slowly.”