13. My friend $has$ $written$ to me about it since yesterday.
$\text{Sử dụng thì hiện tại hoàn thành : S + have/has + V3 . }$
$\text{dấu hiệu : '' since yesterday''}$
14. The final examination has $been$ $taken$ by all students since yesterday.
$\text{Sd thì HTHT (dạng bị động ) : S + have/has + been + V3 + by + O}$
$\text{dấu hiệu : '' since yesterday''}$