Trying to figure out why in the first example the verb in the dependent clause (après que ...)  is in the passé composé, but in the next two examples the verb following après que is in the present.   All three examples seem similar in that the first action is completed before the action in the independent clause.    Is the difference that the final two examples express habitual actions, as mentioned in the explanation?  (Though the first example seems like it could express a habitual action as well).  I guess in English we could say either, "After they've arrived, they go and say hello to my mother" or "After they arrive, they go and say hello to my mother," so maybe it's a matter of choice whether to use the passé composé or the present (après qu'ils sont arrivés or après qu'ils arrivent; après qu'elle a sonné la cloche or après qu'elle sonne la cloche)??