> Could and might are often, but not always, interchangeable. He might have come / studied = 'We don’t know whether has come / studied or not', but you could substitute could. If you want to indicate ability or permission, however, you need He could have come / studied = ‘He had the ability to come / study.'
> It's possible that he came from a different culture would be used in considering a historical figure who didn’t seem to fit into his environment. If you were talking about a current situation, you’d say It's possible that he comes from a different culture. It’s hard to think of any circumstances in which It's possible that he come from a different culture would be used, even if you regard ‘come’ as subjunctive.
> English modal verbs are both important and subtle. You're unlikely to gain a full understanding of their use in exchanges such as this and if, as I assume, Noah, you are a non-native speaker of English you really need the help of a qualified English teacher.
[0]: http://dictionary.cambridge.org/us/grammar/british-grammar/m...
[1]: http://www.learnersdictionary.com/qa/modal-verbs-may-might-c...
[2]: https://english.stackexchange.com/questions/64374/might-have...
[3]: https://english.stackexchange.com/questions/99957/what-is-th...
[4]: https://english.stackexchange.com/questions/4661/it-could-mi...
[5]: https://www.myenglishteacher.eu/question/would-could-should-...
[6]: https://forum.wordreference.com/threads/could-vs-might.25206...
[7]: http://www.talkenglish.com/grammar/auxiliary-verbs-can-could...
[8]: https://www.englishforums.com/English/CouldVsMight/blmxzx/po...
EDIT: Quote from [2]:
> Could and might are often, but not always, interchangeable. He might have come / studied = 'We don’t know whether has come / studied or not', but you could substitute could. If you want to indicate ability or permission, however, you need He could have come / studied = ‘He had the ability to come / study.'
> It's possible that he came from a different culture would be used in considering a historical figure who didn’t seem to fit into his environment. If you were talking about a current situation, you’d say It's possible that he comes from a different culture. It’s hard to think of any circumstances in which It's possible that he come from a different culture would be used, even if you regard ‘come’ as subjunctive.
> English modal verbs are both important and subtle. You're unlikely to gain a full understanding of their use in exchanges such as this and if, as I assume, Noah, you are a non-native speaker of English you really need the help of a qualified English teacher.