Learn Finnish: Munkkihan pidättäytyy kaikesta ylellisyydestä. - The monk, as you know, abstains from all luxury.

The verb pidättäytyä (to abstain, refrain) belongs to the family of verbs expressing refusing, declining, or separating yourself from something. Just like kieltäytyä (to refuse), it requires the thing being avoided to be in the elative case (-sta/-stä). Because kaikki modifies ylellisyys, they both must take the elative ending.

Pidättäytyä looks like a very long verb. Is it related to a simpler root?

Yes, it is built on the root verb pitää (to hold). From there, we get pidättää (to hold back, to retain, to arrest). By adding the reflexive ending -ytyä, we get pidättäytyä, which literally means "to hold oneself back" (to abstain). Recognizing these suffixes helps unpack long, complex Finnish verbs.

Why does the stem of ylellisyys (luxury) change to ylellisyyde- before adding the -stä ending?

Nouns ending in -uus or -yys change that ending to -uute- or -yyte- to form their basic stem. When a closed syllable ending like the elative -stä is added, consonant gradation kicks in and weakens the strong t into a d, resulting in the stem ylellisyyde-.

What is the purpose of the suffix on ?