Student or Learner
I advised him to focus in/on his studies that helps/help him to improve the marks.
Should I choose 'in' or 'on' ?
Should I choose 'helps' or 'help'?
Please explain the reason so I can apply in the future. Thanks.
"On". The simple reason is that the standard phrase is "to focus on something/someone".
Use "help" in this case. The relevant word is "studies" so "he has to focus on studies that help him improve his marks".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.