while pretending to stretch:是状语从句的省略， 连词后省略they were,又如： e.8 a.If (it is)carefully done,the experiment will be successful. 如果小心，试验会成功的。 b.Don't speak until (you are)spoken to. 别人不跟你讲话，你就不要跟别人讲话。while pretending to stretch是状语从句的省略， 连词后省略they were, 又如: e.g. a. If (it is) carefully done, the experiment will be successful. 如果小心，试验会成功的。 b. Don’t speak until (you are) spoken to. 别人不跟你讲话，你就不要跟别人讲话