Rule: If the verb in a reporting clause is in the past tense, the verb in the reported clause is past or past perfect, unless it expresses something that is still true at the moment of speaking.
Examples of errors:
(1) She said I can eat as much as I want.
(2) He said that he has a stomach ache.
(3) So I was astonished when I learned that I'm able to study in Kassel.
Verbs that typically appear in reporting clauses (i.e. the main clause that introduces reported or indirect speech) are say, tell, know, think, learn, realize, forget, remember, etc. In indirect (reported) speech the verb in the subordinate (reported) clause is backshifted--shifted back in time--relative to the tense that would occur in direct speech (i.e. a direct quote). (1), in direct speech, would be
She said: "You can eat as much as you want."
If the tense in the reported or quoted clause is past or present perfect in direct speech, it becomes past perfect in reported or indirect speech:
She said: "You have eaten/ate it all!"
= She said I had eaten it all!
Back-shifting is optional if the reported clause can be assumed to be as true at the moment of speaking as in the past. For example, one can say
She said someone is/was at the door.
'if it is still possible that someone is at the door. It'it is no longer possible that anyone is at the door, only was is acceptable.
The German speaker has two problems with interference from rules for tense usage in indirect speech in German. First, the choice in German is only partly dependent on the relation of the time of reference of the verb in the subordinate clause to the time of speaking; it also depends on the speaker's attitude toward the truth or validity of the subordinate predication. Thus, to take Nehls' (1978:63-64) examples, the choice in
Peter sagte mir, daß er krank ist/sei/wäre.
depends on to what extent the speaker is convinced that Peter is really sick--but all referring to his state at the present moment. If the reference is to Peter's state of health in the past, there is again a further choice to make, depending on how convinced the speaker is that Peter was really sick:
Peter sagte mir (letzte Woche), daß er krank war/gewesen ist/gewesen sei/gewesen wäre
Secondly, we must take into account the fact that colloquially speakers use the forms'in the first sentence for both present and past time reference, even when the reported verb refers only to the past, e.g.:
Peter sagte mir (letzte Woche), daß er krank ist/sei/ware.
This is probably the source of error in (l)-(3), namely:
Sie sagte/hat gesagt, ich kann so viel essen wie ich will.
Er sagte (hat gesagt), er hat Bauchweh.
...als ich erfahren habe, daß ich in Kassel studieren kann.