Learn Finnish: Pako oli todella vaikea. - The escape was very difficult.

It shares a root with the verb paeta (to escape or flee). Finnish often creates nouns from verbs by taking the verb stem, sometimes applying consonant changes, and adding an -o or -ö ending. Another common example is teko (act or deed) from the verb tehdä (to do).

Could I use tosi instead of todella in this sentence?

Yes, you could definitely say Pako oli tosi vaikea. Both words mean "really" or "very" in this context. Tosi is slightly more informal and heavily used in spoken Finnish, while todella is standard and fits perfectly in both written and everyday language.

Why is vaikea in the basic nominative form instead of the partitive form vaikeaa?

We use the nominative vaikea because the subject, pako (the escape), is treated as a single, countable event with clear boundaries. If the subject were an uncountable concept or a continuous activity (like pakeneminen, meaning "escaping"), the adjective would have to be in the partitive case: Pakeneminen oli vaikeaa.