Usually, nouns that end in -o are masculine and nouns that end in -a are feminine. For example:
There are some exceptions to this rule, eg planeta, día, mapa and tranvía are all masculine, whereas mano, foto and radio are all feminine.
Nouns with these endings are usually masculine:
Nouns with these endings are usually feminine:
There are always exceptions to these rules. It can be hard to tell if some nouns with other endings are masculine or feminine. For example, nouns that end in -e could be either masculine, eg hombre or feminine, eg leche. It's better to check in a dictionary than to guess.
Are these nouns masculine or feminine?
Make these nouns plural.