Have you ever come up with this such big question, (either as a learner or as a teacher)? Internal/external obligation, laws, rules, etc.
Grammar books and webpages are so confusing when dealing with this matter, that we should pay more attention to the actual usage of the language
and try to get the support of native speakers for these things.
This post is just for you to check out this Wordreference Forum thread, and draw your own conclusions.