This is the 125th most frequent Finnish word.
usein
Often.
Here 'usein' is an adverb used to describe the frequency of the action 'käyn' (I go), indicating it occurs frequently.
Käyn usein kirjastossa.
I often go to the library.
Here 'usein' modifies the verb 'sataa' (it rains), expressing regular or habitual occurrence.
Usein sataa kun lähden ulos.
It often rains when I go outside.
In this usage, 'usein' describes the frequency of the state of being 'iloinen' (happy), applied to the subject 'hän' (she).