Write a letter to your friend telling him what have happened .
=Write a letter to your friend 【and tell】 him what have happened
句意：給你的朋友寫封信,告訴他已經(jīng)發(fā)生過的事情.
或 給你的朋友寫封信【同時(shí)/并且告訴】他已經(jīng)發(fā)生過的事情.
telling 是v-ing,做狀語,表示與write 這一動(dòng)作相伴隨的動(dòng)作.
telling 【him間接賓語】【 what have happened直接賓語】 這叫【v-ing 形式短語】,其構(gòu)成是：
【v-ing+ 賓語或狀語】