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17 You can go when you've finished your work. In adverbial clauses of time referring to future, we use one of the present tenses. We do not use shall or will in a predictive sense in such clauses. The conjunctions commonly used to introduce time clauses are: when, as soon as, before, after, untill, by the time (that), the moment (that). In some sentences, there may be little difference in meaning between the present simple and the present perfect in the time clause: 18 I shall leave as soon as the meeting ends has ended. In other cases, however, there are two factors that appear to operate in determining an English speaker's choice: <1> The perfect form indicate that an action or event occurs before the time indicated by the context. We can, therefore, distinguish between: 19a Come over and see us when our guests leave. 19b Come over and see us when our guests have left. Sentence 19a might suggest that the arrival of one set of visitors will coinside with the leaving of the other, whereas 19b clearly indicates that our guests' will no longer be with us when our other visitors come. A great deal depends on the meaning of the verb involved. Either the present perfect or the present simple is appropriate in 18 above, while only the present perfect makes good sense in 20: 20 We can go out as soon as we've had dinner. (not We can go out as soon as we have dinner.) <2> The perfect forms also indicate that an action or event has produced a result or state of affairs that is relevant to the current situation, and there is often a causal connection between the time clause and the main clause: 21 You'll fell a lot better after you've had a rest. The conjunction after itself clearly extablishes the time relationship between the time activities of events, and the present perfect is used here to establish the causal connection between these events rather than to establish a time difference between them. |