Learn Finnish: Koko maata ei liene valloitettu yhdessä yössä. - The whole country was probably not conquered in one night.

This uses the negative potential perfect passive. You use ei liene, which is the negative potential form of the auxiliary verb olla, followed by the passive past participle of the main verb, valloitettu (from valloittaa, to conquer). The potential mood here adds the meaning of 'probably'.

Why is the object koko maata in the partitive case instead of the nominative?

In a standard affirmative passive sentence, a complete object would be in the nominative case (for example, koko maa on valloitettu). However, because this sentence is negative due to the word ei, the object must follow the negative object rule and take the partitive case, becoming koko maata.

Shouldn't the adjective koko also be in the partitive to match maata?

Normally, adjectives agree with the case of the noun they modify. However, koko (meaning 'whole' or 'entire') is a special indeclinable word in Finnish. It never changes its form, regardless of what case the noun is in.

Why is the inessive case used for the time expression yhdessä yössä (in one night)?

When you want to express the time frame within which an action is completed, Finnish uses the inessive case (the -ssa/-ssä ending). This directly matches the English concept of completing an action 'in' a certain amount of time.