最后,the bottom line is - verb tense is a meaning-based issue.
最后的最后,引用Ron大神的话—— ** WARNING ** for non-native speakers of english (as well as every other language in the world -- this is not a unique feature of english), verb tenses are MUCH harder than any other aspect of the language. in fact, it's almost impossible for any non-native speaker of any language to master verb tenses fully and completely. even when an intelligent speaker has been using his or her second language exclusively for decades, he or she will still routinely make verb-tense errors.
therefore: DO NOT study verb tense until you have 100% MASTERED the other MAJOR sc topics. * parallelism * pronoun use * subject-verb agreement * modifier placement if you cannot identify these error types literally one hundred percent of the time, then do not study other error types -- especially verb tense (which should be the absolute bottom priority for non-native speakers) -- until you can.