ΤΟ ΟΡΟΣ the hill, the mountain

 
τὸ ἔτος, τοῦ ἔτους (the year)
τὸ ξίφος, τοῦ ξίφους (the sword)
τὸ στῆθος, τοῦ στήθους (the chest lesson 6b, doctors apply stetho-scopes to our chest)  
τὸ τέμενος, τοῦ τεμένους (the temple, lesson 9a)
τὸ ἄνθος, τοῦ ἄνθους (the flower, lesson 10b3, my notes)
 
These neuter nouns in -ος look more irregular than they really are:
  1. They take the normal, basic 3rd declension endings
  2. They follow the usual rules for stem ε contracting with endings:
    • The genitive should be: τοῦ ὄρεσος (stem: ὄρεσ-) but
      1. the end σ- was dropped, leaving
      2. ὄρε- and as ε + ο contract to ου , we have:
      τοῦ ὄρους instead of ὄρεος.
    • And in endings ε + α contract to η, so the plural will be τὰ ὄρη instead of ὄρεα
 
Singular
  nominative          τὸ    ὄρος    καλόν ἐστιν
  genitive   οἱ       τοῦ    ὄρους     λύκοι σπάνιοί εἰσιν. (rare)
  dative   ἐν     τῷ    ὄρει     λαγώς εἰσιν.
  accusative   ὁρῶ     τὸ    ὄρος     καὶ θαυμάζω (wonder, admire)
  vocative     ὦ      ὄρος,     καλὸν εἶ
 
Plural
  nominative             τὰ    ὄρη     καλά ἐστιν
  genitive   τὰ     τῶν    ὀρῶν     δένδρα (trees)
  dative   ἐν     τοῖς    ὄρεσι     λαγώς διώκομεν
  accusative   θαυμάζω     τὰ    ὄρη   καλὰ γάρ ἐστιν
  vocative     ὦ      ὄρη,     ὑψηλά ἐστε (tall, high)
 
Now think of real-life situations involving mountains or hills, seeing them, falling down, skiing, photographing, pictures thereof seen in books, on TV, in films, people living there etc.

What about trying out the following. In context, mind you, we want to use the whole of our brain, not just the lefthand purely intellectual part.
τὸ ἔτος, τοῦ ἔτους (the year)
τὸ ξίφος, τοῦ ξίφους (the sword)
τὸ στῆθος, τοῦ στήθους (the chest lesson 6b, doctors apply stetho-scopes to our chest)
τὸ τέμενος, τοῦ τεμένους (the temple, lesson 9a)
τὸ ἄνθος, τοῦ ἄνθους (the flower, lesson 10b3, my notes)