알맞다 - it is like "suitable" in English. Also you can use this word when something is not too much / not little. Like a English word "enough". Ex. 알맞은 답을 고르세요. 키에 알맞게 의자 높이를 조절하다. 걸맞다 - when situations/ things can be comparable and fit in together. Ex. 졸업식에 걸맞은 옷을 입고 가야해요. I tried my best to explain.. it might be not very clear for you but hopefully you get the idea!:)