Learn Finnish: Tämä uusi tuote toimii täydellisesti.

The base adjective is täydellinen (perfect). To make it an adverb, you attach the -sti ending to the word's stem. Words ending in -nen change to -se- in their stem. Therefore, täydellinen becomes täydellise-, and adding -sti creates täydellisesti (perfectly).

Why is the verb toimia used here for "works" instead of työskennellä?

English uses "work" for both jobs and machines, but Finnish splits them up. Työskennellä means "to do work" (like a person doing a job). Toimia means "to function" or "to operate" (like a machine, a product, or a plan). Since we are talking about a product functioning, we use toimia.

Why does toimii end with a double i?

The dictionary form is toimia (verb type 1). To conjugate it for a "he/she/it" subject (in this case, "the product"), you take the stem toimi- and lengthen the final vowel. This gives us toimii.

Tuote (product) ends in -e. What happens to it if we need to add location endings like "in the product"?