Advice leaflets originally produced for the Study Advice Service in the University of Hull, which holds the copyright:
Loan-words from other languages tend not to keep their native forms in English - except among specialist students, e.g. of languages. (As in this leaflet, the use of original forms is often a form of showing off. But it is showing off that impresses academics. So it is recommended to you.) In modern European languages, French nouns and adjectives have regular plurals, like English, with -s. Some whose singulars end in -u form plurals in -ux. Sometimes, where the masculine ends in -if, the feminine ends -ive (e.g. naif/naive).
Italian is a language that has given us many of our words to do with music and the visual arts. Many end in -o in the singular. In Italian, most become -i in the plural. English musicians are not consistent in the way they have used these terms. If you are studying music, you'll pick up the difference in usage as you go along - I trust! Others might notice that cello can be pluralised as celli or cellos; we have sets of concerti (← concerto); more than one piccolo are piccolos, never, to my knowledge, piccoli in English; and so on.