Latin has four distinct kinds of interrogative adjectives each with its own separate function in asking questions about a noun in a sentence. Just like any type of adjective, Latin interrogative adjectives must agree with the nouns they modify in case, number, and gender. Recall that the noun and the adjective that modifies it may not necessarily have the same form.
The simplest employment of interrogative adjectives in Latin occurs when the question about the noun asks “what.” For this type of question, “qui, quae, quod” (literally “what”) is used. For example:
Quam bacam in agro agricola crescit?
What fruit is the farmer growing in the field?
Latin students often wonder why “quam bacam” is in the accusative case. It is because “bacam” is a direct object in the sentence and “quam” is simply agreeing with the noun it modifies in case, number, and gender. By rearranging the translation of the sentence from Latin to English, its function as a direct object becomes easier to see. For example:
The farmer is growing what fruit in the field?
Although this is a non-standard form of a question in English, this arrangement clearly shows that “farmer” is the subject to the verb phrase “is growing” with “what fruit” acting as the direct object, hence the accusative case.
Latin can also ask “what kind of” questions using interrogative adjectives. There are two ways to express this type of question. One is with the use of “qui, quae, quod” again. For example:
Quae baca in agro crescit?
What kind of fruit is growing in the field?
Notice that “quae baca” is the subject of the sentence and is, therefore, in the nominative case. Also, notice that the interrogative adjective “quae” agrees with the noun it modifies “baca” in case, number, and gender but not form.