qui

The relative pronoun, and along with "sum" the thing that decides whether a page of Latin opens up or stays shut. The commonest forms are "qui" (masculine, doing the verb), "quae" (feminine, doing the verb), "quod" (neuter), "quem" (masculine, on the receiving end) and "cuius" (whose). Add "quibus" for "to whom" or "by whom" in the plural and you can read a great deal.

The rule that unlocks it is a split one. GENDER and NUMBER come from the noun it points back to, so a feminine singular antecedent forces "quae" or "quam". CASE comes from the job the pronoun does inside its own clause, which is why "vir quem vidi" (the man whom I saw) has an accusative even though "vir" is nominative.

Two traps worth flagging early. "quod" is also a conjunction meaning "because", and "quam" is also an adverb meaning "than". Context, and the noun it agrees with, settle both.