Questions about the Subjunctive
Q: Why is it Quiero que tú salgas and NOT *Quiero que tú sales?
Remember, the stem is always based on the present tense yo form. Since the present of salir is yo salgo, the stem for the subjunctive is salg-.
Q: Why is it ... después que lo hago and NOT ... después que lo haga?
The expression después que is followed by the indicative, while antes que is followed by the subjunctive. This makes sense because we can be more certain about events after they happen than before. With después que, it is implied that the event occurred, so the indicative is used.
Q: Why is it creo que viene and NOT creo que venga?
The verb creer is followed by the indicative in affirmative (positive) sentences, but followed by the subjunctive in negative ones (the idea being that there is less certainly in the latter case), e.g.: No creo que venga.
Q: Why is it nopienso que lo sepa and NOT no pienso que lo sabe?
The verb pensar is followed by the subjunctive in negative sentences and by the indicative in affirmative (positive) ones (the idea being that there is certainly/assertion, rather than doubt, in the positive case), e.g.: Pienso que lo sabe.