Student or Learner
does both refers to two guys? or it can be more.
Can I say to group of my firends:
You guys both are excelant.
2) Is it the same if I say ask teacher and ask a teacher?
Not a teacher.
1) Both is a word that refers to only two things. If you said that to a group of friends, they would start wondering which two you were talking about! If you wanted to refer to the group, you would have to use "all" or just say "you guys are excellent."
2) If you say "ask a teacher" then you are talking about asking some non-specific teacher. If you say "ask teacher" then you are referring to a specific teacher, whose name you replaced with "teacher." Little kids usually say it this way while older people will probably say "ask the teacher" instead.