"Since" indicates the starting point in time, and is usually used with present perfect: - I have been studying English since 2015. - I've studied English for two years. - He has lived here since 1997. - He's been living here since before the turn of the millennium. "Ago" indicates the point in time that something happened, measuring back from the present moment. It's used with the past tense: - I started studying English two years ago. - He moved here twenty years ago. Because "ago" doesn't indicate whether the action has continued to the present, another clause is often added using "since." - I started studying English two years ago, and kept up with it ever since. - He has been a good neighbor ever since he moved here twenty years ago.

The right why to put those sentences is: I have been studying English for 2 years. He has lived here for 20 years. Or "He has been living here for 20 years". A good use of "since" would be: I have been studying English since 2015. He has been living here since he was born.

I have been studying English for 2 years. He has lived here for 20 years. / He has been living here for 20 years.