Learn Finnish: Ohennatko sinä minun hiuksiani saksilla? - Do you thin out my hair with scissors?

It is derived from the adjective ohut (thin). To create a verb that means 'to make something [adjective]', we use the suffix -ntaa / -ntää. First, we take the inflectional stem of ohut, which is ohen-. Adding the suffix gives us the transitive verb ohentaa (to thin something out).

Why is hiuksiani in the partitive case here instead of the accusative?

Hair on a head is treated as a plural noun in Finnish (hiukset). When the verb ohentaa is used with a plural mass like hair, the action doesn't logically 'finish' or consume the entire object in a bounded way. Because the action affects an indefinite amount of the hair, we use the partitive plural form hiuksia (which then gets the possessive suffix -ni appended to it).

Could I drop the word minun and just say Ohennatko sinä hiuksiani saksilla?

Yes, absolutely. In standard Finnish, first- and second-person genitive pronouns (minun, sinun, meidän, teidän) are optional if the noun has the correct possessive suffix attached. Since hiuksiani already contains the first-person singular suffix -ni, the word minun can be safely dropped.

Why is 'with scissors' translated as saksilla without using a separate word for 'with'?