Ystäväni ostaa aina matkamuiston, vaikka matka olisi lyhyt.

Ystäväni is built from:

• ystävä = friend
• -ni = my

So ystäväni means my friend.

A useful extra point: in some contexts, ystäväni can also mean my friends, because possessive forms can blur the singular/plural distinction. But here the verb is ostaa, which is singular, so the sentence clearly means my friend, not my friends.

Because Finnish often shows possession with a suffix attached to the noun.

So instead of needing a separate word, Finnish can simply say:

• ystäväni = my friend

You can also say:

• minun ystäväni

That is also correct, but it is more explicit or emphatic. In everyday spoken Finnish, people often use forms like:

• mun ystävä

So this sentence uses a standard written-style possessive form.

This is a very common source of confusion.

For many Finnish type 1 verbs, the:

• 1st infinitive = dictionary form
• 3rd person singular present = he/she buys

look exactly the same in writing.

So ostaa can mean either:

• to buy
• buys

Context tells you which one it is. Here, after the subject ystäväni, it is clearly a finite verb:

• Ystäväni ostaa = My friend buys

The basic form is matkamuisto.

Here it becomes matkamuiston because it is the object of the verb, and it is understood as a whole, completed item: one souvenir.

In learner-friendly terms, Finnish often uses this -n form for a total object in singular active sentences.

So:

• ostaa matkamuiston = buy a souvenir, as a complete item

If you used matkamuistoa instead, that would be the partitive, which would give a different nuance, such as:

• an incomplete action
• an indefinite amount
• a less bounded object

In this sentence, matkamuiston is the natural choice.

Yes. Finnish forms compounds very freely, and they are usually written as one word.

Matkamuisto is a compound:

• matka = trip, journey
• muisto = memory, keepsake

Together, matkamuisto means souvenir.

Then the case ending is added to the whole compound:

• matkamuisto → matkamuiston

Vaikka is a conjunction. Here it introduces a concessive clause.

Depending on context, it can mean things like:

• although
• even though
• even if

In this sentence, because of the conditional olisi, the meaning is closest to even if.

So vaikka matka olisi lyhyt means something like:

• even if the trip is short
• no matter if the trip is short

Olisi is the conditional form of olla.

That matters because:

• vaikka ... on often sounds more factual: although it is
• vaikka ... olisi often sounds more general or hypothetical: even if it were / even if it is

So:

• vaikka matka on lyhyt = although the trip is short
• vaikka matka olisi lyhyt = even if the trip is short

In your sentence, the speaker is describing a general habit: the friend buys a souvenir regardless of trip length. That is why the conditional fits well.

In matka olisi lyhyt:

• matka is the subject
• lyhyt is a predicate adjective after olla

With a singular subject in this kind of clause, both normally appear in the nominative:

• matka = trip
• lyhyt = short

So the structure is basically:

• the trip would be short

There is no special object case here, because this is not an object phrase. It is a subject + olla + predicate adjective structure.

Because Finnish normally uses a comma between a main clause and a subordinate clause.

Here:

• Ystäväni ostaa aina matkamuiston = main clause
• vaikka matka olisi lyhyt = subordinate clause

So the comma is standard punctuation.

You can also put the subordinate clause first:

• Vaikka matka olisi lyhyt, ystäväni ostaa aina matkamuiston.

That is also correct.

Yes, Finnish word order is fairly flexible, but the original order is the most neutral and natural here.

The sentence starts with the subject:

• Ystäväni

then the verb:

• ostaa

then the adverb:

• aina

then the object:

• matkamuiston

This is a very normal structure.

You could also say:

• Vaikka matka olisi lyhyt, ystäväni ostaa aina matkamuiston.

That just moves the subordinate clause to the front. The meaning stays the same, but the emphasis changes slightly.