If you're saying "(Under) this/that/these/those ..." (Where "Under" can be replaced by any preposition), you say the preposition twice; once after the demonstrative pronoun (e.g. "That"), and again after the noun. If the preposition starts with a vowel, you don't need to do anything:
- Az alatt a fa alatt - Under that tree
- Ez előtt a ház előtt - In front of this house
If, however, the preposition starts with a consonant, "Az" and "Ez" lose their final -z:
- E mellett a folyó mellett - Next to this river
- A mögött a hegy mögött - Behind that mountain
Note that "Azok" and "Ezek" remain unchanged, whether the preposition starts with a vowel or not:
- Azok belül a négyzetek belül - Within those squares
- Ezek után az emberek után - After these people
In contrast to the excellent and complex conversation regarding the twice appearing "alatt" - I thank you all for it - I have just this to add: "What is underneath the car" is an equally good translation, in my view. I've submitted it for consideration. Curious if our little feathered teacher (Professzor Bagoly) will accept it.
This may be what those double case endings / postpositions are for :)
Once you see az alatt, you know it must be "under that ....", so az alatt az autó alatt can only mean "under that car" and not "that under the car" (which, I think, would be az, ami az autó alatt van "that which is under the car").
Oh, I think I understand your question now. The reason we can tell (from the hungarian sentence) is az alatt means under that.
Remember that suffixes apply to both the demonstrative ez,az as well as the noun: abban az autóban, "in that car", and alatt is similar. So az alatt az autó alatt as a whole can only mean "under that car". If the az alatt and az autó alatt were somehow separate, then the sentence would be more like "what is under that under the car" which doesn't make as much sense.
I think the way to write what you've written is more like Mi az van az autó alatt but I am not sure if that is correct/natural.
- az autó = the car
- az az autó = that car
- az autó alatt = under the car
- az alatt az autó alatt = under that car
In each case, the first az in the sentences with two of them is the demonstrative "that" (and the second one with the definite article "the", which English does not use in such a situation).
That would be wrong -- in a similar way to saying "that big desks".
In English, you have to pluralise not only "desk" but also "that", and it becomes "those big desks".
And in Hungarian, you have to put the postposition alatt not only after az autó but also after az, and it becomes az alatt az autó alatt.
If you only do half the work (only one postposition), it would be a bit as if you did only half the work of pluralising in English.