if you check the controls u will see buttons for previous attack modifer and next attack modifier. buttons are "." and "," what is the use of this?
i just dont understand

you can only use one attack modiifer at a time, so if u buy two you have the option to choose which one u want to use (for example if u buy frostburn+shieldbreaker) u can either choose to use the frostburn slow or the -armor from shieldbreaker

how about if i activated elder parasite then witch to shield breaker? is that possible?

i assume you want the ias buff from elder..
so yes, that will work. you wont get lifesteal from it tho.