SUBJECT + 为 (wéi) + AGENT + 所 (suǒ) + VERB With this passive sentence pattern, you shouldn't add 了 after the verb. Please have a look at this quote: 卫太子为江充所败 。 Wèi tàizǐ (SUBJECT) wéi Jiāngchōng (AGENT) suǒ bài (VERB). ‘The Prince of Wei (SUBJECT) was defeated (VERB) by Jiangchong (AGENT).’ Now, let's look at your example sentence again: "森林为大火所毁。" Sēnlín (SUBJECT) wéi dà huǒ (AGENT) suǒ huǐ (VERB). "The forest (SUBJECT) was destroyed (VERB) by the large fire (AGENT)."

我猜，其實也沒有很確定。according to the dictionary of Ministry of Education, R.O.C., "為" should means reasons; Because of fire, the forest had been dystroied. Actually, it's a little like passive tense, but it's not, I guess. If so, that make sense. 森林因大火而毀、國家因民富而強、天下因太平而安，"為...所..."應該等於"因...而..."，如果是"因...而..."，那不家"了"就說得通了，雖然他的意思有點和"被"相近。你可以看一下辭典解釋：http://dict.revised.moe.edu.tw/cgi-bin/newDict/dict.sh?cond=%AC%B0&pieceLen=50&fld=1&cat=&ukey=2085061048&serial=4&recNo=451&op=f&imgFont=1

As a native speaker from Shanghai.I just want to say.There is very little tense in Chinese.So 森林为大火所毁is correct，森林为大火所毁了 is also correct.But the best and simple way is 森林被大火毁了or 森林被大火所毁，森林被大火烧了。We use 被。 give you an example: present tense:We usually use 在。我在看书。I am reading .(usually we dont add 了 at the end of the sentence.) When your mom asked you "are you reading?"you can answer “我在看书了”

森林为大火所毁,There isn't omited "了"。森林为大火所毁了---it's incorrect. we can say:森林被大火所毁,森林被大火毁了,森林为大火所毁。 they are both mean: "The forest was destroyed by the large fire." “被”=by. native speakers always said:森林被大火毁了。

Woops , didn't mean to put the 的 there XD -->謝謝你們都回答我!