Are these sentences correct? If not, please help me with them. Thanks.
11) Your parents always understand your problems. Or Your parents have always understood your problems.
12) The patient has been taking penicillin for several days.
13) A big earthquake did not hit San Francisco since 1906.
14) They have been living in San Francisco since they arrived in USA twenty years ago.
15) He got ill five weeks ago and has not recovered yet.
Those are all OK except #13. Have another try there. The trigger word for the tense is "since".
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Those are all OK except #13. Have another try there. The trigger word for the tense is "since".
On a separate point, in #14 "in USA" should be "in the USA".