Learn Finnish: Värjään hiukset ja laitan raidan tuohon. - I will dye the hair and put a highlight in that.

The word tuo means "that". To say "into that" (where you are putting the highlight), we use the illative case. For the pronoun tuo, this case is formed by adding -hon, making it tuohon.

Why is "hair" translated as hiukset?

In English, "hair" is usually a singular, uncountable word. In Finnish, the hair on your head is treated as a plural word, hiukset. A single strand of hair would be hius.

Why is the word for highlight raidan instead of raita?

The basic word is raita, which literally means "stripe". It is the object of the verb laitan (I put). Because you will completely finish putting the highlight in, this singular object takes the accusative ending -n, making it raidan.

Why doesn't hiukset have an -n ending like raidan if it's also an object?

For plural words, the total object (which shows a completed action) looks exactly like the basic plural form ending in -t (hiukset). The -n object ending is only used for singular words.