Japanese grammar question?

Question:

1.　is there a difference between かなり, けっこう, and なかなか？

2. 店では買いません。 don't quite understand what function は serves?

as usual, thank you.

Answer:

1. for adverb use (they can be adjective by adding "-na")
For example, you take an exam, and later you say;
かなり難しかった。It was quite difficult.
けっこう難しかった。It was quite difficult (unexpectedly)
なかなか難しかった。 It was quite difficult (but it sounds the most objective of the 3)

2. 店では買いません。 ネット・ショッピングの方が便利ですから。
I don't buy those at stores. I'd rather go for net-shopping becasuse it's handy.
That is, for emphasizing or there's other options of where to buy.