Dear Justine Marie, It's totally clear to me now. Your comprehensive explanation is quite useful. Thank you very much. ^__^

In modern conversational English both are accepted. Until can be used with most tenses. Until marks a point in time when something will happen/stop happening. All sentences from S1-S4 are correct. S2 and S4 have quite similar meanings in conversation. The only small difference is that if you use Present Perfect the action has for sure been completed or perfected. "Stir until the sugar has dissolved would mean that the processes of dissolving is complete. (There is no sugar in solid form left, because the dissolution is total and complete) If you use "Stir well until the sugar dissolves" you are sure that the process of dissolution has started BUT you are unsure if it is complete (But in conversation this does not matter because dissolving sugar does not take too much time anyway so this difference becomes inconsequential and the meanings in these 2 sentences merge into the same thing.) This is the same with S1 and S3. There's really no great time difference between the time the subject 'starts to hear' from the object and when he 'finishes hearing' from him. And if there is it's probably just the amount of time he takes to read an email/message/ answer the phone that will prompt him to stop waiting. The differences between Simple present and present perfect can be seen in the other functions of the tenses. For example: I eat rice. (This is Simple present to denote a RECURRING TRUTH/FACT that has happened before, is true now and will happen again in the future.) I have eaten rice. (This is Present perfect used to denote an EXPERIENCE that has been perfected/completed in the past but is still true now)