The best way to understand Greek prepositions and their objects is

by way of the 'Space-Time continuum', as follows:

1) any preposition showing motion toward a goal or object will take accusative endings (like those of direct object), -ον , etc.

2) any preposition marking 'place where' (X marks the spot, not going anywhere) will take dative endings (like indirect object),   -ῳ, etc.

3)  any preposition showing motion from or out of source or origin will take genitive endings (like possessive), -ου, etc.

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                        πρός                 ἐν                        ἐκ/ ἐξ

------------------------------>   X  -------------------------->

    -ον                                   -ῳ                        -ου

 

 

So, for example, to(ward) the house is  

------------------->                        Χ                ----------------->

πρὸς τὸν οἶκον               out of the house is

                                                                       ἐκ τοῦ οἴκου

In the house is         ἐν τῶι οἴκῳ