Ostan purkin jogurttia iltapalaksi.

Ostan is the first person singular form of the verb ostaa (to buy).

• ostaa – basic infinitive form (to buy).
• ostan – minä-form (I buy / I am buying).
• The personal ending -n marks the subject I:
  
  • (minä) ostan – I buy
  • (sinä) ostat – you buy
  • (hän) ostaa – he/she buys

Finnish usually drops the pronoun (minä) because the verb ending already shows who is doing the action, so Ostan alone means I buy / I’m buying.

Purkki is the basic (nominative) form jar, can.
Purkin is the genitive singular, and in this sentence it functions as a total object.

In Finnish, a complete, whole object of a completed action is often in the genitive:

• Ostan purkin. – I’ll buy a jar (one whole jar).
• Ostan kirjan. – I’ll buy a book (one whole book).

If you said Ostan purkki, it would be grammatically wrong here. For a normal, complete “I buy a jar”, purkin is the correct form.

So Ostan purkin… = I’ll buy a (whole) jar…

Jogurttia is the partitive form of jogurtti (yogurt).

Here it describes the contents of the jar: “a jar of yogurt”. In Finnish, the content of a container is very often in the partitive:

• lasillinen maitoa – a glass of milk
• pullo viiniä – a bottle of wine
• purkki jogurttia – a jar of yogurt

The partitive is used because yogurt is treated as a mass / indefinite amount rather than one countable item. You’re not buying “one yogurt-thing”, but a jar containing some yogurt.

The main object of the verb ostan is purkin.

Grammatically:

• Ostan purkin. – I buy a jar. → purkin = object
• purkin jogurttia – “a jar of yogurt” → jogurttia just describes what is in the jar.

So the structure is:

• Ostan [purkin [jogurttia]].
  
  • Verb: ostan
  • Object: purkin
  • Contents of the object (mass noun in partitive): jogurttia

Iltapalaksi is iltapala (evening snack) in the translative case (-ksi).

The translative often means:

• “for (the purpose of) X”
• “to become / as X”

In this sentence:

• iltapalaksi ≈ for an evening snack / as an evening snack

So Ostan purkin jogurttia iltapalaksi means I’ll buy a jar of yogurt for (my) evening snack.

If you used other cases, the meaning would change:

• iltapalaan (illative: into the evening snack) – sounds odd in this context.
• iltapalalle – literally “onto/to the evening snack”; not used this way.

The natural way to express “for evening snack” is iltapalaksi.

Iltapala is a compound of:

• ilta – evening
• pala – literally piece, bite, small portion

Together, iltapala means a light meal or snack eaten in the evening, often later in the evening, not a full dinner.

So iltapalaksi = for an evening snack (meal).

Yes, that is possible and correct, but the meaning changes slightly:

• Ostan purkin jogurttia iltapalaksi.
  – I’ll buy a jar of yogurt for evening snack. (one jar, specified quantity)

• Ostan jogurttia iltapalaksi.
  – I’ll buy (some) yogurt for evening snack. (unspecified amount, maybe from a bigger container, or in general)

Without purkin, the focus is on the substance (some yogurt), not on the exact package.

Finnish has no articles like English a/an or the.
Definiteness and indefiniteness are shown by context, word order, and case, not by separate words.

So:

• Ostan purkin jogurttia. can mean:
  • I’m buying a jar of yogurt.
  • I’m buying the jar of yogurt.

Which one is meant is understood from the situation or previous sentences, not from extra words like a/the.

No, both are wrong in normal standard Finnish in this meaning.

• Ostan purkki jogurttia – incorrect, because purkki should not be in nominative as a total object.
• Ostan purkkiä jogurttia – also incorrect; purkki in partitive (purkkiä) would suggest an incomplete amount of jar, which doesn’t make sense.

For “I’ll buy a jar of yogurt” with a complete action and one whole jar, you need:

• Ostan purkin jogurttia.

You can change the word order quite flexibly in Finnish, and the sentence remains grammatical:

• Ostan purkin jogurttia iltapalaksi. – neutral order.
• Iltapalaksi ostan purkin jogurttia. – emphasizes for evening snack.
• Purkin jogurttia ostan iltapalaksi. – emphasizes a jar of yogurt.

Case endings (like -n, -a, -ksi) tell you the grammatical role, so word order is used mostly to change emphasis and style, not basic meaning.