One of my colleague asked her students to rephrase this sentence:
We started to write to each other three years ago.
And her answer is:
We used to write to each other in two years.
I see something wrong in her answer, is it right? Because “used to” is used for an action in the past and it no longer happens. I can rewrite this like that:
We have written to each other for three years.
I have been writing to each other for three years.