I was told that both of them mean 'really' or 'very' but I am still confused on how to use them in a very natural way in conversation. And what is the difference would it make if I use 아주,엄청, or 너무 instead ?
굉장히 is definitely a stronger intensifier. It can be stronger than 'very', and could even be 'extremely'. I would translate 꽤 as "quite" or "a little more than normal" - stronger than "a little", but not "very".
굉장히 추워요 it's extremely cold
꽤 추워요 it's quite cold
There are quite a few other ways to modify the intensity of adjectives. Here are some others:
- 아주 - very (this feels quite neutral to me)
- 매우 - very (sounds more formal, often used in written language)
- 엄청 - very (sounds stronger, but very informal)
- 너무 - too. Also used to mean 'very'. (Grammarians will say it is incorrect to use this as 'very', but in actual usage, it is very common to hear people use it this way, as in "날씨가 너무 좋다!" ="The weather is really nice!" - this is common in spoken language).
- 정말 - really (very informal when used to modify adjectives, as in 정말 맛있다!)
- 진짜 - really (informal when used to modify adjectives,as in 진짜 어렵다!)
- 몹시 - very (stronger than 아주; it sounds more formal or literary to me, but I'm not sure)
- 되게 - very (used in spoken language)