Question

一个句子里面可以时态混用吗？比如这句

举个自己写的例子:
The program introduced me to a variety of approaches to clean energy utilization, but I think they cannot solve the problems thoroughly; the nuclear energy would offer the only answer to the world’s long-term energy needs, thus I determined to concentrate on it.

这个本科专业向我介绍了各种能源清洁利用的途径（这是发生在过去的），但是我认为它们不能够彻底解决问题，唯有核能可以满足世界未来长期的能源需求（这是现在也存在的一般性想法），所以我决定从事这个领域（这是在过去决定的）。

Answer 1

一个句子里时态是可以混用的，但是不能像你写的这样混用。你应该把I think 改成 I thought they couldnot……
正确的混用方法，举个例子吧，在虚拟语气中，有错综时间条件句。
They would have finished the program more quickly but i was really busy.
他们本来可以更快地完成项目的（在我的帮助下），但是我实在是太忙了。

Answer 2

My major introduced me a variety of approaches to clean energy utilization, but I thought they cannot solve the problems thoroughly. Because the nuclear energy will offer the only answer
that the long-term energy needs of the world's. In that time, I determined to concentrate on nuclear energy.