まきさんはなんねんせいですか?
--> In which / what grade Maki-san is? (The example answer is in the second grade now.)
かよってるだいがくはどこですか?
--> What is the university which you go to?
せんこうはなんですか? --> What (subject) is your major?
(*せんこうするは --> せんこうは)
けんさんは23さいです。--> Ken-san is 23 years old.
けんさんはよんねんせいです。--> Ken-san is in the fourth grade.
(*よんねんせい --> よねんせい)
けんさんのだいがくはにほんだいがくです。--> The university which Ken-san goes to is Nihon University.
けんさんのせんこうするはにほんごです。--> Ken-san's major is Japanese / Ken-san is a Japanese major.
I hope this was helpful and if my English isn't correct, I'd like you to know the correct ways to say.