I study English = a fact, or a regular habit. I'm studying English = at this moment in time / presently (this implies the action will stop at some point). I've been studying English = I began studying in the past, and continued right up to now (ie. so far... the "-ing" form suggests I will continue my study into the future). For comparison, "I studied English" means you don't do this any more.

I've been studying значит, что вы уже какое-то время изучали и все еще продолжаете изучать, т. е. процесс когда-то начат, но не закончен, поэтому да, если вас спросили, сколько времени вы изучаете английский, то отвечаете I've been studying. Если вы просто говорите, например, рассказываете о себе, вы говорите I study: I go to school, I study English there... I'm studying только если вы изучаете в данный момент времени.

I study English. I would explain with an example. You are in a college at present and your major is English. If some one asks you; What are you studying in the college? You can reply; I study English. I have been studying English. This is a past tense in which "study' was a continuous process. But at present you no longer are doing the same. If you need further clarification, you can refer to Tenses.