Learn Finnish: Vanha ammattilainen on usein viisas. - An old professional is often wise.

The translative case (ending in -ksi) is used when someone or something changes into a new state or role (for example, Hän opiskeli ammattilaiseksi - He studied to become a professional). In this sentence, there is no change happening; we are just stating what the person currently is. Therefore, we use the basic nominative case.

How is the word ammattilainen formed?

It is built from the noun ammatti, which means "profession" or "trade". By adding the suffix -lainen (which refers to a person belonging to a certain group, place, or type), you get ammattilainen — literally a "person of a profession", or a professional.

Why is the adjective viisas in the basic nominative form instead of the partitive (viisasta)?

In Finnish "A is B" sentences, when describing a person or a single countable object, the describing adjective stays in the nominative case (viisas). You would use the partitive (viisasta) if you were describing an uncountable substance (like water) or making an abstract statement (like On viisasta... -> "It is wise to...").

Where else can the adverb usein go in this sentence?

Finnish word order is quite flexible, so you could also start the sentence with it: Usein vanha ammattilainen on viisas. This puts slightly more emphasis on the word "Often". However, placing it right after the verb on is the most neutral and natural spot for it.