主要 as adverb?? or is it a sub+pred as subjet phrase
Fix #1

'Mainly' = '主要是'. The 是 is omitted 

补习老师[主要是]帮她练习什么? 

(What does the tutor [mainly] help her practice?)

Also, it would be more specific to use "补习老师的工作" or "补习老师的責任" as the subject instead of just  "补习老师"


  Q: "What is the tutor's job mainly help her practice?" (补习老师的工作主要是帮她练习什么?)
  
  A: "The tutor's job is mainly help her practice English." (补习老師的工作主要是幫她練習英語)


Fix #2

Using 主要 as an adjective:


  补习老师的主要工作是帮她练习什么?
  
  补习老師的主要工作是幫她練習英語


In this case, 主要(main) would be the adjective for the noun 工作 (job) and you have to add 的 after 补习老師 to indicate possessive