SINGULAR
  
nominative           ἡ    πόλις   μεγάλη ἐστίν.
genitive  ἀπὸ     τῆς    πόλεως     ἄγγελος ἥκει.
dative     τῇ    πόλει     προσχωροῦμεν
accusative  ὁρῶσιν     τὴν    πόλιν     καὶ θαυμάζουσιν.
vocative   ὦ      πόλι     καλὴ εἶ.
  
PLURAL
  
nominative         αἱ    πόλεις     μεγάλαι εἰσίν.
genitive  οὐ πᾶσαι αἱ     τῶν    πόλεων     οἰκίαι καλαί εἰσιν.
dative ἐν    ταῖς    πόλεσι     σπανίως ὁρῶμεν αἶγας. 
accusative   ζητοῦμεν     τὰς    πόλεις     τὰς τῶν Ἑλλήνων on maps 
vocative   ὦ      πόλεις    you've gone but your glory remains.
  
 
Now think of real-life towns. What comes to mind? description, seen, want to go there, who lives there? anything is better than nothing at all. You've GOT to practice those forms actively, not learn them passively off by heart!!!

Likewise (all feminine, all in -εως ):

When I said "ALL" I certainly did not mean "including men",
ὁ Δικαιόπολις, τοῦ Δικαιοπόλεως  is still masculine of course
ἡ Ἀκρόπολις, τῆς Ἀκροπόλεως 
ἡ πανήγυρις, τῆς πανηγύρεως 
ἡ πρᾶξις, τῆς πράξεως 
ἡ ἀνάλυσις, τῆς ἀναλύσεως 
ἡ κρίσις, τῆς κρίσεως 
ἡ φύσις, τῆς φύσεως 

and plenty more