an important thing to remember is that in passive clause, "to do" do not refer to the subject of the clause
this pen is used to write on the white board
. in this sentence, somebody write. the pen dose not write.
this thing is burried to keep it from stealing
somebody keep it from stealing. this thing dose not keep.

contrast, active clause, "to do" refer to the subject of the clause.
I learn gmat to go to us. subject "I' will go.

this point is simple but and grammatically. but if we do not know how can we write with confidence?