Either

1.    Conjunction  :  either + or = one or the other of two

  Either Sam or Joe will help you.

  He is either in London or in Paris.

  You must be either for or against the ideas.

  Come either today or tommorrow.

  Either you or he is to go.

  Either he or you are to go.

2.    Pronoun   :  either = one or the other  of two

   2.1    Either as the subject of the sentence.

   Either will be good enough.

   Either can sing the song.

   2.2    Either + of + piural noun (pro)

   Either of the buys will help you.

   Either of the cats is playing.

   Either of them is good enough.

 * NOTE  :  Negative sentences - I don't think....... ; I don't agree ...

   Plural verb is used with either.

   I don't think either of them are at home.

3.    Adjective   :  either = one of the other of two; both

   Either boy will help you. = Both boys will help you.

   You may go by either road. (by both roads)

   There are shops on either side. (on both sides)

   There are trees on either side of the river.

4.    Adverb  :  either = any more than another (negative meaning)

    Sam won't help you and joe won't either.

    Somboon isn't a swimmer and Somsak isn't either.

    He doesn't study and I don't either.