Ostin matkamuiston torilta.

Ostin is the 1st person singular past tense of ostaa = to buy.

• ostaa = to buy
• ostin = I bought

In Finnish grammar, this past tense is often called the imperfect.

Finnish often leaves out the subject pronoun when it is clear from the verb ending.

Here, ostin already tells you the subject is minä = I.

So:

• Minä ostin matkamuiston torilta.
• Ostin matkamuiston torilta.

Both mean the same thing, but the version without minä is very natural and common. You would add minä mainly for emphasis or contrast.

Because it is the object of the sentence, and in this kind of sentence Finnish uses the total object.

Ostin matkamuiston suggests buying one whole souvenir, as a completed action.

In the singular, this total object often looks like the genitive form, so:

• matkamuisto = souvenir
• matkamuiston = the form used here as a total object

Many learners notice that this looks like genitive, and that is normal. In traditional descriptions, the singular total object is often said to have a form identical to the genitive.

Matkamuistoa would be the partitive form.

Finnish often uses the partitive object when the action is:

• incomplete
• ongoing
• negative
• or referring to an unspecified amount

But Ostin matkamuiston presents the buying as a completed event with a whole, countable object.

So the contrast is roughly:

• Ostin matkamuiston = I bought a souvenir / one complete souvenir
• En ostanut matkamuistoa = I didn’t buy a souvenir
• Ostin matkamuistoja = I bought souvenirs

Torilta is the ablative case.

It means from a place, especially from an external location.

With tori = market square / marketplace, the local case pattern is:

• torilla = at the market
• torille = to the market
• torilta = from the market

So torilta tells you where the souvenir was bought from.

Because tori is normally treated as an open area / surface-like location, so Finnish uses the external local cases:

• torilla
• torille
• torilta

The ending -sta / -stä usually means out of the inside of something:

• talosta = out of the house
• kaupasta = out of the shop / from the shop

A market square is not thought of as an enclosed inside-space, so torilta is the natural form.

Yes. Matkamuisto is a compound word, which is very common in Finnish.

It is made from:

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

Together, matkamuisto means souvenir.

In Finnish compounds, the last part is the part that usually inflects:

• matkamuisto
• matkamuiston
• matkamuistot

Finnish has no articles like English a/an/the.

So matkamuiston does not itself tell you whether English should use:

• a souvenir
• the souvenir

That depends on the context. Finnish usually expresses definiteness through context, word order, or other words if needed.

No, Finnish word order is fairly flexible, although some orders sound more neutral than others.

Ostin matkamuiston torilta is a very natural, neutral order.

You could also say:

• Torilta ostin matkamuiston.
  This emphasizes where I bought it from.

• Matkamuiston ostin torilta.
  This emphasizes what I bought.

So the basic meaning stays similar, but the emphasis changes.

Usually tori means an open market square or marketplace.

Depending on context, English may translate it as:

• market
• market square
• marketplace

So torilta can be understood as from the market or from the market square.

A natural negative version is:

En ostanut matkamuistoa torilta.

Notice two important changes:

• ostin becomes en ostanut
• matkamuiston becomes matkamuistoa

This is very typical in Finnish: with negation, the object often changes to the partitive.

So:

• Ostin matkamuiston torilta. = I bought a souvenir from the market.
• En ostanut matkamuistoa torilta. = I didn’t buy a souvenir from the market.