も

も replaces は or が to add the meaning "also." 私も学生です — "I am also a student" (presupposing someone else already mentioned is). It can also stack onto other particles (にも, でも, とも) to add "also" without replacing the case marker: 東京にも行った — "I also went to Tokyo."

With quantifiers, も means "even" or "as many as": 三時間も待った — "I waited a full three hours" (suggesting that's a lot). With negative predicates, も in quantifier position becomes "not even one": 一人も来なかった — "not even one person came."

Doubled も creates an exhaustive "both X and Y" or "neither X nor Y": 寿司も天ぷらも好き — "I like both sushi and tempura." In negative contexts: 寿司も天ぷらも食べない — "I eat neither sushi nor tempura."