Learn Finnish: Hän matkustaa harvoin.

While many adverbs are formed by adding -sti to an adjective, words about time and frequency often have their own special forms. The adjective harva means sparse or few, but its adverb form is harvoin (rarely or seldom), not harvasti. Another common frequency adverb that breaks the -sti rule is usein (often).

Why is harvoin placed after the verb, unlike in English?

In English, frequency adverbs like rarely often go before the verb (He rarely travels). In Finnish, the basic word order is subject-verb-adverb, so harvoin typically comes directly after the verb. Saying Hän harvoin matkustaa would sound like an unnatural English translation to a native speaker.

Just as a refresher, why does matkustaa have a double A at the end?

This is the third-person singular (he/she) form. For this basic verb type, you form the he/she present tense by taking the verb stem, which is matkusta-, and doubling its final vowel.