also, too, as well, and even
도 is used after subject and object nouns to express the listing of subjects and objects or the addition of a subject or object to one mentioned previously. This meaning corresponds to 'also' or 'too' in English.
When added to a subject particle, the subject particle is omitted, leaving only 도.
Similarly, when added to the object particle, 을/를 is omitted, leaving only 도.
When 도 is added to particles other than subject or object particles, however, the other particle is not omitted.
It indicates that the noun is being added to a list of items, actions, or descriptions previously mentioned or understood from the context.
When combined with 은/는 (Topic) or 을/를 (Object) the 도 particle can replace them entirely, rather than appending to them.
In other cases, it functions as usual, adding its meaning without replacing any existing particles