一个句子里可以出现两种时态，，例如
一、l will be a doctor when l grow up.(我长大了要当医生) 例句中，主句l will be a doctor是一般将来时，从句when l grow up是一般现在时。
二、He was singing when l came(我来时他正在唱歌). 例句中，主句He was singing是过去进行时，从句when l came是一般过去时。
一个句子可有两种时态真正的含义是：一句话中可以描述两个时空不同的事件
如：
I guess (that) his faith helped him face the challenge.
I think (that) his talents came from his mother’s side of the family.
这两句话都是说话当下的想法或认知状态，用I guess/I think，属于一般现在时。
但想法的内容却是关于过去发生的事，所以补语子句中用过去式helped him/came from。属于一般过去时。
以下两句话，子句中不同的时态was/is表达出信念存在的阶段：
I thought English was difficult, but it is not. →过去的信念，现在不认为如此
I thought English is difficult and still do. →对事实的认定，一向认为如此
一个单句只能有一个时态。
如：I am reading
一个复句（主从句）有两个以上的时态。
如：I'm sorry （主句）to hear that you resorted to cheating（从句）
