The nominative case is the case for the subject of the sentence. The subject is the person or thing about which the predicate makes a statement, and the name, "nominative," means "pertaining to the person or thing designated." Thus, you could say "Mary fired Joe" and the subject would be "Mary," the person designated as the actor in the sentence; but, if you said "Mary was fired by Joe," "Mary" would still be the subject, but this time designated as the one "acted upon." In Latin the subject does not always need to be expressed because it can be indicated by the person and number of the verb. "They praise" = laudant. See the subject case in English, which is similar to the Latin nominative case. You may want to think about what case is in Latin (and in other languages); some general observations can be found here.