Both "I've only had the car for eight days" and "I only have the car for eight days" are accepted, even though there is a big difference in meaning between them. So my question is: Is the Czech sentence really a perfectly fine translation of the simple present, or should it be something like "Mám to auto jen na osm dnů."?
Because you specify the time, there is hardly any difference in the English meaning of 'have had' and 'have'. You got the car 8 days ago and you still have it. Most native speaker would use 'have had' here. Czech does not have ability to use tenses and the present is the best translation.
Note: Mám to auto jen NA osm dnů" is not the same thing. That means that you have the car for 8 days only. Today might be day 2. You still have 6 to go.
Mám to auto jen osm dnů" means you got the car 8 days ago, you have only had it for those 8 days (and now it needs service already or so)-
Note the difference here:
1. "I have had the car (for) 8 days " means I got the car 8 days ago and still have it.
2. "I have the car (for) 8 days'" means I have the use of the car for a period of 8 days.
So, for example:
1. I've had the car only 8 days, and already it needs service.
2. We had better start our vacation now, because I have the car only 8 days.
True, not below ;)
And true, the exercise introduces some confusing English grammar subtleties that, while helpful for English speakers for understanding what Czech does and does not mean, might not be an ideal exercise for the next round of improvements. Thanks for updating though, it was clearly incorrect as it was.