The plural is formed like nouns. In the masculine gender, we create it by replacing the endings –o, -e, -a with –i:

alto - alti

buono - buoni

felice - felici

entusiasta - entusiasti.

In feminine gender, however, there is a change from –a to –e, -e to –i:

alta – alte

buona – buone

felice – felici.