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to study - изучать, учиться
to learn - изучать, учить (что-то)
to teach - учить (кого-то)

If you have a direct object (for example "I'm studying (learning) Russian" here "Russian" is the direct...

изучат means the future action - they will study (investigate) the circumstances. While изучают means that they're doing it now. изучать can't be used here Американцы изучать is impossible.