Contains simply means that they are in there. Includes means that although there may be other things there, there are also these items. It is the opposite of excludes. With the word 'include', it can sound more purposeful. For example: "Make sure that the sentence you write includes also an adjective." versus "Many sentences contain adjectives." Here is another example: "We placed an order for 10 pizzas, and we made sure that the order included also a pizza without tomatoes because two of the staff members are allergic to tomatoes."

It depends on the greater context. A sentence could contain adjectives in the positive degree, but it might also include adjectives in the positive degree if that was your purpose in writing it. To say something like, "I have a sentence that includes adjectives in the positive degree" sounds a little off to me, I would use contain instead here. But, "I wrote a sentence that includes adjectives in the positive degree" is acceptable.

I don't understand the question.