Rule: In the if clause of real. conditions, the simple present tense is used for future time reference; otherwise, tense usage is the same as in other sentence types.
Examples of errors:
The most common type of real conditional sentence has the simple present (referring to future as well as present time) in the if clause and will + infinitive in the main clause, but almost any combination of tenses is possible in real conditional sentences--except that will does not normally occur in the if clause with future time reference. If will does appear in an if clause, it has-the special meaning of 'be willing to'. often used in polite or softened suggestions such as If you'll clear the table, I'll wash the dishes. Another rare use of will in if clauses occurs in sentences like
If it'll bore you, then let's not go.
Here, as Palmer (1974:148.) points out, the time reference of the verb in the if clause is understood as subsequent, to that of the main verb. The sense is perhaps easier to understand by the paraphrase -which follows, showing that will derives from the main clause (it will bore you) of an underlying real conditional sentence: if (it will bore you if we go) then let's not go. In any case these uses of will in if clauses are relatively uncommon and do not justify will in (4)-(ll). The restriction on will future in if clauses also applies to clauses introduced by whatever (cf. (9)), whoever, however. wherever, and whenever, which are special types of conditional clauses (cf. Quirk et al. 1972:§11.36).
Otherwise the rules of tense usage are the same in real conditions as in other kinds of sentences. For example, a sentence like
If somebody asks me a question, I try to answer it.
is a statement of universal truth--i.e. according to the author at least, it is true at all times--equivalent to
Whenever X is the case (i.e. somebody asks me a question) then Y is also the case (i.e. I try to answer it).
(7), however, the author is predicting a future situation given a certain condition--it is not true at the moment of speaking:
If X becomes the case in the future (i.e. the Kernstudium contains Fachdidaktik), Y will also become the case (i.e. you won't have time for other subjects).
The predicted future events or situations require a future tense, although Rule 2.5.1 allows the simple present in the special instance of the if clause.
The error in (8) is describable as misuse of the progressive (cf. 2.3); the verb in question simply happens to occur in an if clause. This is not to say that the progressive cannot occur in if clauses. For example:
If he's thinking about his marital problems, he has difficulty expressing himself.
Here the progressive has a time-frame effect (cf. 2.3-5). In (8), though, the author intended a cause and effect relationship between the if clause and the main clause, as in
Can't you think of anything?
Well, if I think about it I guess I can.
The problem in (3) and (4) is probably confusion of past perfect with simple past (cf. Rule 2.4-3), rather than confusion with past unreal conditions (cf. Rule 2.5.2), but perhaps is a combination of both. In any case, these are not unreal conditions, since they both are intended to express real questions about past events, and the 'previous-to-past' meaning of the past perfect is not appropriate, because it is not clear what past events these predications are supposed to be considered previous to. Again, this doesn't mean that such a combination of tenses in a (real) conditional sentence is not possible. Compare:
If Mary had already heard the joke, then she's a very, polite person, because she laughed anyway.
Here the past perfect has the same 'previous-to-past' meaning as in other sentences--i.e. 'If she had already heard the joke (before you told it to her)...
It is possible for can and the other modal verbs to have future as well as present time, reference, because it is interpretable as 'to be in a state of being able (or of having permission or the opportunity)--now--to do something later. So one can say to a patient in a hospital, for example,
You can go home tomorrow.
meaning 'You are now well enough to go home tomorrow'.
In a sentence like (1), however, can is not interpretable in this way, since-the if clause makes it clear that the state of ability to imagine will not exist until after the condition is fulfilled, and so an explicitly future tense form is required.