Common case slips
Ten minutes on the topic's own four common mistakes: a feminine -а noun left in the nominative where it stands as an object, an accusative form wrongly used as the sentence's own subject, an ending invented for an inanimate masculine noun that needs none, and the nominative reached for instead of the vocative when calling out to someone. Build sentences around loving and wanting something, catching each slip in turn, before the unit's own listening and reading put the whole topic to work.
The part of the grammar page this lesson drills — read it here, then use it.
Nominative vs. accusative - the pair you need now
The nominative is the subject form and the dictionary form used in the noun-gender lessons.
The accusative marks the direct object - what the verb's action lands on. Feminine nouns ending in -а take the ending -у: кућа → кућу, жена → жену, књига → књигу.
Inanimate masculine and neuter nouns are the same in the singular - nominative and accusative share the same ending. Animate masculine nouns follow a different rule, covered later.
Special cases
The vocative is for direct address only. Many masculine names stay the same (Марко!), while most feminine nouns change: мама → Мамо!
Кога/шта имаш? takes the accusative, since имати always needs a direct object (see verb-imati-present-a1).