Learn Finnish: Käsky hyökätä oli virhe. - The order to attack was a mistake.

Certain abstract nouns in Finnish, like käsky (order), päätös (decision), or aika (time), can govern an action directly. This attached action takes the first infinitive (the standard dictionary form), mirroring the English structure in phrases like "the order to attack".

Could I express this using a compound word instead of separating the noun and the verb?

Yes, you could simply say Hyökkäyskäsky oli virhe (The attack order was a mistake). Finnish uses compound nouns very frequently. However, using the abstract noun plus the infinitive structure (käsky hyökätä) places slightly more emphasis on the action itself.

If I want to add who was ordered to attack, where does that go in the sentence?

The recipient of the order usually goes between the abstract noun and the infinitive, typically in the allative case. For example, Käsky joukoille hyökätä oli virhe means "The order for the troops to attack was a mistake".

As a quick refresher, why is virhe in the nominative case and not the partitive (virhettä)?

With the verb olla (to be), the predicative (the word describing the subject) is nominative if the subject is a countable, individual thing. Even though käsky is an abstract noun, it acts as a singular, countable concept here (one specific order), so virhe must take the nominative case.