1.The widely accepted big bang theory holds that the universe began in an explosive instant ten to twenty billion years ago and has been expanding ever since.
在这里用begins 表示一个科学事实似乎也说得通 而此处用began是为了强调产生这个动作所以用过去时？
在另一个题里又看到OG里讲
(D) psychologists declared that since most of the studies fail in controlling for such variables like social class and family size,
English has a rule of sequence of tenses: once a verb form is marked for past tense, the following verb forms that describe the same object or event have to be in the past tense as well. Thus, fail is the wrong verb form.
下面是正确答案 请问这里该怎么理解呢？
2. In a review of 2,000 studies of human behavior that date back to the 1940s, two Swiss psychologists declared that since most of the studies had failed to control for variables such as social class and family size , none could be taken seriously.
The subject two Swiss psychologists is followed by a verb in the past tense (declared); the dependent clause since ... family size also has the correct verb form (had failed).