Thank you so much.I don't think there's any situation in which you have to say "more busy" rather than "busier".
The only situation in which I think "more busy" is likely is when it's preceded by "even": even more busy. But even then, "even busier" is perfectly acceptable/normal.
If I were you, I'd stick with "busier".
Can you tell us what are the cases that we should say "more busy" instead of "busier" ?There are few contexts where "more busy" is an acceptable alternative to "busier". I suggest that you always use "busier" - you will never be wrong in doing so.
Thank you, can I say " I know that you are busy and having a pet will make you more busy" ? Is that right?I can imagine (but with some difficulty) a situation in which "busier" would not work and one would have to use "more busy". It would probably involve different attributes or qualities of a person, comparing how busy they are with how XXXX they are, only I can't think of a suitable specific example for XXXX.
If the question isn't really about "busier" vs "more busy", but generically about "XXXXer" vs "more XXXX", then consider this example:
We're looking at some object and are disagreeing over what colour it is. In fact it's borderline green/blue, which one might describe as either greeny blue or blueish green. I might say it's blue. You might say "I think it's more green than blue". "Greener" would not work here. Although it's possible for something to be greener than blue (if you consider blue to have certain amount of greenness), that wouldn't mean quite the same thing. Here you would be expressing that its colour is closer to green than it is to blue, but something that is merely greener than blue could well still be closer to blue than to green.