for作为介词不可以加句子

这是白勇里面说的，可是OG里面一道题是这样的

The Olympic Games helped to keep peace among the pugnacious states of the Greek world in that a sacred truce was proclaimed during the festival's month.

(D) world, for a sacred truce was proclaimed during the month of the festival.

答案选D。解释甚至说the conjunction for joins the two clauses correctly and economically.

???

那什么时候能够知道for是作为连接词还是介词呢？

是不是介词的时候当“为了”讲，连接词的时候当“因为”讲？

一個簡單的判斷技巧:

for 作介係詞時，接名詞；作連接詞時，接句子。