Learn Hardcore Finnish: Veljeni on sinkku ja naimaton, mutta ystäväni on ollut naimisissa jo kauan. - My brother is single and unmarried, but my friend has been married for a long time already.

sinkku
- Means single (usually about relationship status).
- Colloquial / everyday, a bit informal.
- Focuses more on not being in a relationship or not being married.
naimaton
- Literally unmarried.
- More neutral or formal.
- Focuses specifically on legal marital status, not on dating or relationships in general.

Saying Veljeni on sinkku ja naimaton repeats the idea in two slightly different ways, a bit like saying in English My brother is single and unmarried. It can sound a little emphatic or slightly humorous in Finnish, because the meanings overlap a lot.

Why is it on sinkku ja naimaton but on ollut naimisissa? What does on ollut mean?

on = is (3rd person singular of olla, to be).
on ollut = has been.

On ollut is the Finnish present perfect tense:

ystäväni on ollut naimisissa = my friend has been married.

So:

Veljeni on sinkku ja naimaton → a present state (is single and unmarried).
Ystäväni on ollut naimisissa jo kauan → a state that started in the past and continues until now (has been married for a long time).

Why is naimisissa in that strange form? What case is it, and why not just something like naiminen?

Naimisissa is a fixed, idiomatic form:

Morphologically it looks like naimis-
- -ssa (inessive plural: in marriage).
You almost always talk about being married using this inessive plural form.

Key patterns:

olla naimisissa = to be married
- Hän on naimisissa. – He/She is married.
mennä naimisiin = to get married (illative plural)
- He menevät naimisiin. – They are getting married.

You do not normally say olla naiminen or similar. Naimisissa is simply the conventional way Finnish expresses the state of being married.

Why are sinkku and naimaton in the basic form (no endings)? What case is that?

They are in the nominative case.

In sentences with olla (to be), the thing you are is usually in the same basic form (nominative) as the subject. This is called a predicative:

Veljeni (subject, nominative)
on (verb olla)
sinkku ja naimaton (predicatives, nominative)

So:

Veljeni on sinkku. – My brother is (a) single (person).
Ystäväni on opettaja. – My friend is (a) teacher.

You only add other cases (like -na, -nä, -ssa) if you want to add specific meanings such as as, in, like, etc.

What is the difference between veljeni and ystäväni compared to minun veljeni and minun ystäväni?

veljeni = veli (brother) + -ni (my) → my brother
ystäväni = ystävä (friend) + -ni (my) → my friend

The suffix -ni is a possessive suffix meaning my.

You can also add the pronoun minun in front:

minun veljeni
minun ystäväni

Both versions are correct:

With only -ni (as in veljeni, ystäväni) → neutral, very common.
With minun
- -ni → often adds emphasis (a bit like my own brother, my friend in contrast to someone else’s).

In many everyday sentences, Finnish speakers just use the suffix and drop minun unless there’s a special reason to emphasize.

Could the sentence use just minun veljeni instead of veljeni? Would that change the meaning?

Yes:

Minun veljeni on sinkku ja naimaton, mutta ystäväni on ollut naimisissa jo kauan.

This is grammatically correct and means essentially the same thing.

Subtle differences:

Veljeni … ystäväni …
- Neutral, compact, typical written style.
Minun veljeni … minun ystäväni …
- More emphatic: my brother vs. my friend.
- The pronoun minun draws attention to the possessor (you).

In normal, neutral narration, the shorter veljeni / ystäväni is more natural.

How does mutta work here? Could I use ja instead?

mutta = but
ja = and

In the sentence:

Veljeni on sinkku ja naimaton, mutta ystäväni on ollut naimisissa jo kauan.

Mutta introduces a contrast:

One person is single and unmarried,
The other has been married for a long time.

If you used ja instead:

Veljeni on sinkku ja naimaton, ja ystäväni on ollut naimisissa jo kauan.

…it would sound more like just listing two facts about two people, with less feeling of contrast between them. Mutta is better here because it highlights the difference.

Why is there a comma before mutta in Finnish? Is that the same rule as in English?

Yes, it is very similar:

Finnish usually places a comma before mutta when it starts a new independent clause (a new full sentence part with its own subject and verb).

So:

Veljeni on sinkku ja naimaton, mutta ystäväni on ollut naimisissa jo kauan.

Two clauses:

Veljeni on sinkku ja naimaton
ystäväni on ollut naimisissa jo kauan

They are joined with mutta, and the comma separates them, just as in English with …, but ….

How does jo kauan work? What is the role of jo and kauan, and could I place jo somewhere else?

jo = already
kauan = for a long time

Together:

jo kauan ≈ already for a long time / for a long time now.

Variations in word order are possible:

Ystäväni on ollut naimisissa jo kauan. (very natural)
Ystäväni on jo kauan ollut naimisissa. (more emphasis on the duration)
Ystäväni on ollut jo kauan naimisissa. (also possible; slight shift of emphasis)

All three are grammatical.
The main idea stays the same: the marriage has lasted a long time already. The exact position of jo can change what is being slightly emphasized, but in everyday speech they often feel almost interchangeable here.

What is the difference between kauan and pitkään? Could I say jo pitkään instead of jo kauan?

Both kauan and pitkään can mean for a long time:

jo kauan
jo pitkään

In this sentence you can say either:

Ystäväni on ollut naimisissa jo kauan.
Ystäväni on ollut naimisissa jo pitkään.

Both are natural and correct.

Nuance:

kauan is perhaps slightly more neutral or ordinary here.
pitkään literally relates to long (length), but as an adverb of time it is extremely common and natural as well.

In practice, they are close enough that learners can treat them as synonyms in this context.

Is there any gender information in veljeni and ystäväni, like in English brother vs sister?

veljeni specifically means my brother (male sibling).
ystäväni means my friend and is gender-neutral by itself.

Finnish in general does not mark gender in pronouns or most nouns:

hän = he or she
ystävä = friend (can be male or female)

So from ystäväni on ollut naimisissa jo kauan, you cannot tell whether the friend is male or female unless the context elsewhere specifies it.