Learn Finnish: Voiko kuka tahansa hallita tätä maata?

kuka tahansa translates to 'anyone' or 'whoever'. The word tahansa is a fixed particle that never changes its form. If you need a different grammatical case in a sentence, only the pronoun kuka changes (for example, into the partitive ketä tahansa). Here it is the subject of the sentence, so it stays in the basic nominative case.

Why is the object of the sentence tätä maata in the partitive case?

The verb hallita (to rule or govern) is a partitive verb. This means it always requires its object to be in the partitive case, which is why tämä maa becomes tätä maata.

Why does the sentence start with Voiko instead of the subject kuka tahansa?

To form a yes/no question in Finnish, you add the question particle -ko or -kö to the verb and move that verb to the very beginning of the sentence. Here, the verb voi (can) takes the -ko ending.

Why isn't the main verb hallita conjugated here?

It follows the modal verb voi (from voida, meaning to be able to). Modal verbs in Finnish require the second verb in the chain to be in its basic dictionary form (the first infinitive), similar to how we say 'can rule' instead of 'can rules' in English.