좋아해요 is "I like", straightforward. And it also could mean "I love you"
좋아 is somewhat weaker than above, and means roughly "It's good", "I kinda like it"
~데 conjugation is used when you expect a reaction from the other party. Simply saying "좋은데?" means "That's good, isn't it?"
The ~데 conjugation plays a somewhat different role when put as a "conjunction"
(similar to ~지만)
examples:
- 이 상품은 다 좋은데 가격이 좀 높다. Everything is great about this product, but the price a little bit too high.
- 답장을 준 것은 고마운데 이미 너무 늦은 것 같다. I appreciate your reply, but it's already too late.