Learn Finnish: Kun poika auttoi minua, hän oli ahkera, mutta tänään hän on laiska. - When the boy helped me, he was hardworking, but today he is lazy.

The verb auttaa (to help) is a partitive verb, meaning it always requires its object to be in the partitive case. Therefore, you must use minua instead of the accusative form.

Why do we use kun for "when" instead of milloin?

Milloin is strictly a question word used to ask "When?". Because this sentence is making a statement connecting two ideas ("At the time when the boy helped me..."), you must use the conjunction kun.

How does the verb auttaa change to auttoi in the past tense?

To form the past tense (imperfect) for verbs ending in -aa or -ää, the final vowel is replaced by the past tense marker -i. For the third-person singular (hän), the final vowel is not doubled like it is in the present tense (hän auttaa), so it simply becomes auttoi.

How would I rephrase this sentence to use the relative pronoun joka (who/which)?

This sentence currently links two clauses with kun (when). If you wanted to use joka to say "The boy who helped me was hardworking", you would combine the clauses into one: Poika, joka auttoi minua, oli ahkera.