Which is grammatically correct I didn’t know he played basketball Or I didn’t know that he plays basketball I’m a native english speaker and I know the first is correct, or at least sounds the best, but why isn’t the second correct? “He” is still playing basketball, so why do you use the past tense when it’s supposed to be that he plays basketball before and now. Or is it the imperfect非完成 tense? --------------------------母語者的回答 簡要總結(jié)：時態(tài)前移其實可做可不做，但做更native，主要大量使用在間接引語中。 Both are grammatically correct (if he does indeed still play basketball), but the version with "played" is probably more common and sounds more natural (as you've noticed) The phenomenon here is "backshifting," which means that we can use a past-tense verb ("played") in a subordinate從屬 clause when the verb in the matrix母體基體 clause (the clause containing the subordinate one) is also past tense (or when the matrix clause refers to a past time, even if it doesn't use a past-tense verb).跟隨主句的時態(tài)前移 Backshifting is just a feature of English grammar, and a backshifted past tense does not necessarily refer to the past, though it can, so the sentence with "played" is ambiguous. If the thing is still true (i.e., he still plays basketball), then it's also fine to use the present tense to reflect this. Note that you probably encounter backshifting most often in reported speech: Direct speech: "I **am** sick." Reported speech 間接引語: "She **said** that she **was** sick." (But "... that she **is** sick" is also correct if she is still sick.) Backshifting also occurs from past tense to past perfect完成時: Direct speech: "I **was** sick." Reported speech: "She **said** that she **had been** sick." (But "... that she **was** sick" is also correct.) And from present perfect to past perfect: Direct speech: "I **have been** sick." Reported speech: "She **said** that she **had been** sick." (But "... that she **has been** sick" is also correct.) You can't backshift a past perfect because there's no tense further back than that: Direct speech: "I **had been** sick." Reported speech: "She **said** that she **had been** sick."