Grammar Note: Could you please explain to me how we can discriminate a real situation from an unreal situation.
Son: The X course is better, but the Y is cheaper.
Father: I would rather you tookthe X ourse.
"would rather" is akin to the subjunctive: it refers to a wish, a desire. In other words, it's the father's wish or desire that his son take the X course.
Let's talk about the subjunctive. With the subjunctive, the verb "were", for example, is used. It looks like the past tense in form, but it's not the past tense at all. It's a form of the verb that expresses an unreal, or hypothetical, or couterfactual event. The tense "looks like" it's in the past, but it isn't the past, nor does it refer to the past.
Now, as tdol has already mentioned, you will, indeed, hear speakers today use the present tense with "would rather [someone else]", as in,
EX: I would rather you take the X course. <a present wish/desire>
Whether "take" is grammatical or not in that context is up for debate. It's certainly "acceptable", though. Speakers use it. ;-) Scholars might argue that it's an example of language change. Speakers know the structure "would rather" [someone else] + past tense expresses a present desire/wish and for that very reason they drop the past tense, or what appears to look like the past tense, and replace it with the present tense, giving:
I would rather you took => I would rather you take
The rule, however, is: wish, be it a past one or a present one, plus what appears to be the past tense; e.g., Subjuctive: I wish she were / Indicative: I wish she was.
In short, that speakers are using "would rather" [someone else] + present tense is an example of the indicative: it indicates a present wish. ;-) It's language in change.
Another camp might argue that "took" => "take" is an example of paradigmatic regularization. That's when irregular forms or structures are regularized, thereby producing a more efficient system. For example, why are there two rules, here?
I would rather DO/take
I would rather she DID/took
It's not very efficient, is it? Why not make it easier on the brain? Change DID to DO:
I would rather DO/take
I would rather she DO/take
Again, the Grammar rule is "would rather" [someone else] + past tense. It's a wish, a desire, akin to the subjunctive, so "would rather you took". But . . . language changes, and "would rather you take" is a sign of that. ;-)
Hope that helps.