What is the difference between american atpl theory and UK/european atpl theory.?

It's like night and day. In America you sit a single written exam composed of 100 multiple choice questions covering all subject areas. You need to answer 70 correctly. All the questions and answers are published for your study beforehand. The majority is all review from previous exams for the instrument and commercial pilot ratings. Someone good at rote memorization could pass it in a weekend of diligent study. In the UK you sit 7 exams, each on a separate subject, requiring everything from multiple choice to long answer. The required coursework takes months to complete. If you fail more than two segments, you must resit the entire set of exams again. If you receive an ATPL under the UK system, you can easily get it converted to an American ATPL< assuming you have the requisite experience (there is no "frozen ATPL in the US). If you an ATPL in the US and wish to convert it to the UK equivalent, you would have to sit all the UK exams. In America, they are long on acquired experience. In the U.K., they rely more on theoretical knowledge. Both systems work. Theory is great to a point, but remember, only experience can save your butt in many critical situations.