"I lend money to my boyfriend."
It's essentially the difference between using lend vs lends (nyancat grammar) in that same sentence. Yeah, you can use lends (kasu and don't add masu), but you end up sounding very colloquial and will likely end up offending someone if you use it with someone you don't know.
Its like using lend vs lends in the same sentence. Sure, you can use kasu (no masu at the end) but your speech is then very colloquial and unless the other person has already dropped masu/desu endings or asked you to stop using keigo (respective speech), then you run the risk of offending them.
Because Duolingo teaches polite Japanese. かします is the polite form of かす and that's it. I have got some "basic form" sentences accepted here, too.
The last wovel changes from u to i when going from dictionary form (informal) to masu form (formal). There is no "si" letter though, so it becomes "shi". Similarly, "tsu" becomes "chi". Those are the tricky ones, otherwise it's a pretty straight forward transformation.