Learn Finnish: Presidentti olisi varmasti välttänyt tuon virheen. - The president would certainly have avoided that mistake.

This is the conditional perfect. You form it by using the conditional form of the auxiliary verb olla (to be) — which is olisi for the third person singular here — followed by the past active participle of the main verb (välttänyt).

Why are tuon and virheen in the genitive/accusative case instead of the partitive?

The object case depends on whether the action is complete or ongoing. Here, avoiding the mistake is seen as a complete, successful action. Since the sentence is affirmative and the object is one specific, whole thing ("that mistake"), this total object takes the genitive-looking accusative case (tuon virheen) rather than the partitive.

Why is the adverb varmasti placed in the middle of the verb structure?

In Finnish compound verb structures, like the conditional perfect (olisi välttänyt), it is very common and natural to place adverbs like varmasti (certainly) right between the auxiliary verb (olisi) and the main verb (välttänyt).

Would the word välttänyt change if there were multiple presidents?