Hi Ali, Sorry for the delay in the replying to your mail. First of all, thanks a lot for your patience with me. And the second, here comes the sweet discussion of the influence of the Arabic language over Persian! It's quite interesting for me

Hello Affix, The word "forest" is singular. One forest. So, "is" is the correct word here. Two thirds of the forest is pine trees. I know it might sound a bit wrong because "pine trees" is plural. But I think "is" is correct.

Hi, this is just a cut & paste job from various internet resources: 1 The Concise Oxford Dictionary of English Etymology 2 The Oxford Dictionary of English Grammar 3 Concise Oxford Companion to the English Language 4 The Concise Oxford