ἐν τοῖς ἀγροῖς ἐν ταῖς Ἀθήναις ἐν τῷ οἴκῳ	Where? position in (dative)
ὑπὸ τῷ δένδρῷ	Where? position under (dative)
εἰς τὸν ἀγρόν	Where to? movement into (accusative)
πρὸς τὸν οἶκον	Where to? movement towards (accusative)
ἐκ τοῦ ἀγροῦ ἐκ τοῦ οἴκου	Where from? movement out of (genitive)

In Short:

position (no movement)	dative	-ῳ
movement towards, into, onto...	accusative	-ον
movement away from, out of	genitive	-ου

You'll remember best
if you fix one example firmly in your mind, for instance:

ὑπὸ τῷ δένδρῳ	that's where the slave is asleep
ἐκ τοῦ ἀγροῦ	those stones must be carried out of the field
πρὸς τὸν οἶκον	that's where D walks to after the day's work is done