I usually use git add -p to selectively stage hunks. But in git add -i I think running the patch command does the same thing to get into patch mode.

If patch mode shows you a hunk, and you only want some of the lines you can press s to split into smaller hunks. Then you'll be prompted whether to add each smaller hunk separately.

If you want to stage a change that is on the same line as a change you don't want to stage, or on an adjacent line, then you need to use e to edit the hunk. Git stages whatever changes are left when you're done editing. The file in the working tree on disk is unchanged.