Question

Only if__according to the directions can the medicine be quite effective.

为什么填taken,而不是having been taken

Answer 1

你好。
1 因为强调的是“一贯的，规律的”，所以我们用taken. ==only if it IS TAKEN-------一般不用完成式结构------has been taken / having been taken .
2 having been taken往往与有for-- -- , since-- - - - 这样的时间状语的短语结构一起使用。