Learn Finnish: Kukaan ei ollut varoittanut meitä ennakkoon.

It uses a three-part structure: the negative verb ei, the past participle of the verb 'to be' (ollut), and the past participle of the main verb (varoittanut). Think of ei ollut as 'had not'.

Does kukaan mean 'nobody' all by itself?

Not quite! Finnish uses a double negative structure here. Kukaan is the form of 'kuka' used in negative sentences to mean 'anyone', so you must pair it with the negative verb (ei) to mean 'nobody'. Literally, kukaan ei ollut translates to 'anyone had not'.

Why is the object 'us' in the partitive case (meitä) instead of the accusative (meidät)?

There are two reasons here! First, the object of a negative sentence is almost always in the partitive case. Second, varoittaa (to warn) is a partitive verb, meaning it requires its object to be in the partitive case anyway, even in positive sentences.

Where does the word ennakkoon (in advance) come from?

It comes from the noun ennakko, which means 'an advance' or 'anticipation'. The -oon ending is the illative case (meaning 'into'). Much like in English, adding this directional case creates an adverb that means 'beforehand' or 'in advance'.