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In Keller's Learn to Read Latin:
When a third-declension adjective has three forms in the nominative singular, the vocabulary entry contains the same elements as the entry for a first-second-declension adjective: the masculine, feminine, and neuter singular nominative.
So how can I tell if an adjective listed in a vocabulary is third-declension or first-second-declension? Thanks.
The endings will tell: In singular nominative the feminine ending is -a and the neuter ending is -um in declensions 1 and 2. The same endings in declension 3 are never -a and -um.
The masculine form alone is not enough to tell the difference, as looking at pulcher and acer will show you. Only when you see pulchra & pulchrum and acris & acre can you tell which declension to follow.
There are many possible ways you might formulate a detection rule. What I suggest works, but I'm not saying this is the only one that does. The declension tables will be your guide here. The singular nominative can be a bit irregular in the third declension and sometimes for the second declension masculine, but feminines and neuters of declension 1/2 are regular.
