What I learned here is that:
-It is impossible to answer the question without 1) and 2), it can seem stupid but this can help you to reconsider the question; if you think you are able to answer the question without more information --> you are wrong.
-The two sentences 1) and 2) will never contradict themselves, there is only one solution to the question.
-Never guess that x is positive or negative, or an integer if the question does not state it.

-Practice, we are used to solve PS during our maths courses, DS is something new.
-You can often use the example/counter-example to state that a sentence is not absolute.
-Just build you your own file with all difficult DS questions, you will infer some rules, that's the best way.

If someone has other tips like that, please write them it can be useful if you are in trouble with a hard DS.