Jaka jest różnica między Why don't you think he is a nice guy? i why do you not think he is a nice guy? ?Możesz podać przykładowe zdania.

@chris0108 Yes, in the examples you gave, those sentences have different meanings with one being a recommendation and the other just asking for information. I see the first example you gave as having the same meaning because it doesn't seem normal to recommend someone to think something about a person's personality. I'll give you one more example where the 2 sentences would have the same meaning and one example where they have different meanings and I'll try to use the verb 'think' in both examples.
Example 1 (same meaning): Why don't you think my idea is good? Why do you not think my idea is good? (both of these statements are asking for information. They're not really making a recommendation.
Example 2: Why don't you think about applying to a different university? (This sentence could be understood as giving a recommendation or just as asking for someone's opinion).
Why do you not think about applying to another university? (This question is just asking for information, but not really giving a recommendation).
Ok. I hope that helps. :)