Please check my understanding of simple past/present perfect (continuous) after "since" Hi, dear teachers. Please look at this first http://forum.wordreference.com/threads/since-she-lived-in-london-after-she-moved-to-london.3162623/ I think using present perfect or present perfect continuous after "since" causes ambiguity in meaning so we shouldn't use the two tenses in since-clause. Now I am going to share my newly learned knowledge from this thread with you, by explaining why using those two tenses are not good English. I really hope you could check my understanding. This is what another native speaker told me: 10. He has been in poor health since he smoked some Djarum Blacks. correct 11. He has been in poor health since he has smoked Djarum Blacks. - sounds odd but might be viable 12. He has been in poor health since he has been smoking Djarum Blacks.could be viable but I'd prefer "started smoking DBs"First, according to that thread, "since + simple past(durative verb or state)" is interpretable, even if it might be wrong(please tell me whether it is really wrong or not). The structure seems to suggest that "something has happened after another thing stopped happening". For example, "I haven't heard from her since she lived in London." This sentence means "I haven't heard from her after she moved away from London". As with your sentence10, it seems to mean that "After he quit smoking some Djarum Blacks, he has been in poor health" Second, according to that thread, the reason why using present perfect or present perfect continuous in since-clause causes ambiguity is that a sentence in either tense is about two times: (1) something that has happened in the past described an event or a state which was the case in the past. In other word, the viewer looks at that event or state, which was completed in the past and its effect doesn't extend up until now, from a past perspective. (2) The viewer looks at that event, which has been done in the past and its effect extends up until now, from a contemporary perspective. Because both tenses have these two properties, "since + present perfect (continuous)" can be interpreted in two ways. One is "After the thing began to happen". The other is "After the thing stopped happening". To conclude, we shouldn't use simple past or present perfect (continuous) for durative verbs or states after "since", because it causes ambiguity. Would you please check my understanding? Do I understand it correctly?Think Thank you for your patience.