The relative pronoun in Latin is qui, quae, quod. It has a different forms for masculine, feminine and neuter, for singular and plural, and for nominative, genitive, accusative, dative and ablative. Look up the chart in the resources section and do your best to learn it off by heart. This one is so common that it really will help if you can recall the different forms from memory.
A relative pronoun always has an antecedent, something it refers back to. In “The man whom I love”, whom refers back to the man. This is helpful in Latin, because the gender and number of the relative pronoun will always match the gender and number of its antecedent. If I love one man, the relative pronoun is singular and masculine. If I love three books, the relative pronoun is plural and neuter.
But for the third item, case, the relative pronoun looks not back but forwards. Its case is not determined by the antecedent, but by the job it’s doing in the sentence as a whole. Study these examples and note two things: that qui, quae, quod often works closely in tandem with is, ea, id, and that some forms of this pronoun (like quae) are ambiguous and need to be handled carefully.
viros quos video – quos is masc. and pl. because viros is masc. and pl., but as the object of video it needs to be accusative, so quos.
mulier cuius librum lego – cuius is fem. and sing. because of mulier, but genitive because it expresses possession.
magna sunt ea quae dico – quae is neut. and pl. because ea is neut. and pl.; it’s accusative because it’s the object of dico.
vacca quae ridet – quae is fem. and sing. because vacca is fem. and sing.; it’s nominative because it’s the cow who’s laughing. She is the subject.
Because these distinctions do not exist in English, or have begun to fade , it can be hard for an English speaker to grasp. Don’t be hard on yourself if you only take in the broad outlines now. But look out for relative pronouns in the Latin passages we are reading. Examples will always make things clearer.