1. The reason why + 句子~~~ is that + 句子（……的原因是……）
2. This phenomenon exists for a number of reasons. First, ……Second, ……Third, ... 这一现象存在有很多原因的，第一……第二……第三……
3. For one thing, … For another thing, ...=On the one hand, …On the other hand…一方面……另一方面……
4. I quite agree with the statement that…The reasons are chiefly as follows. 我十分赞同这一论述，即……。其主要原因如下。