According to the end of the adjective there are 4 groups:
GROUP 1. O, OS, A, AS for MS, MP, FS, FP respectively
GROUP 2. E, ES for single and pleural respectively
GROUP 3. L, LES for single and pleural respectively
GROUP 4. L, LES, LA, LAS for MS, MP, FS FP respectively
M: masculine
F: feminine
S: single
P: pleural
Remember that in group 3, the consonant will almost always be either "l" or "s". In group 4, you might find other consonants, such as "n", "r", or "s", as well as "l".
