Plural Forms

Masculine
ὁ δοῦλος οἱ δοῦλοι
ὁ becomes οἱ
ος becomes οι (short οι)
no change in stress

Neuter
τὸ ἄροτρον τὰ ἄροτρα
τὸ becomes τὰ
ον becomes α
no change in stress

ὁ ἀγρός    οἱ ἀγροί

μικροὶ ἀγροί
καλοὶ ἀγροί

ὁ ἄνθρωπος    οἱ ἄνθρωποι

πολλοὶ καλοὶ ἄνθρωποι stars in films for instance

ὁ αὐτουργός    οἱ αὐτουργοί

οἱ ἐλευθεροὶ αὐτουργοὶ χαίρουσιν

ὁ γεωργός    οἱ γεωργοί

ἄοκνοι γεωργοί get up very early

ὁ οἶκος    οἱ οἶκοι

καλοὶ οἶκοι are usually expensive, unless they are haunted of course

ὁ πόνος    οἱ πόνοι

χαλεποὶ πόνοι κατατρίβουσιν wear us out

ὁ βίος    οἱ βίοι of parrots

οἱ μακροὶ βίοι parrots are supposed to live long lives
οἱ ἑπτὰ βίοι the 7 lives of cats

τὸ δένδρον    τὰ δένδρα

τὰ καλὰ δένδρα give beautiful shade
τοσαῦτα δένδρα prevent us from seeing the woods

As you must have found out by now, the preposition ὄπισθεν "behind" takes the genitive form. Despite the fact that it indicates position as well as movement. We've got to be flexible in learning languages, so make the forms sound right!
Accent-wise, because genitive and dative forms end in a long syllable, if that last syllable is stressed/pitched it carries a circumflex, to show that it IS long. If the last syllable is unemphasized, acute accent on the penult.

	ἐν τῷ ἀγρῷ	ἐν τοῖς ἀγροῖς
	ἐν τῷ οἴκῳ	ἐν τοῖς οἴκοις
	ὑπὸ τῷ δένδρῳ	ὑπὸ τοῖς δένδροις
λίθοι	ὑπὸ τῶ ἀρότρῳ	ὑπὸ τοῖς ἀρότροις
	ὑπὸ τῷ λίθῳ	ὑπὸ τοῖς λίθοις     scorpions
	ἐν τῷ κλήρῳ	ἐν τοῖς κλήροις

 

Accusative and nominative forms of neuter nouns are identical. No change of form, no change of accent mark.

Accusative singular words end in ν but the accent stays the same as in the base form.
The plural form ους counts as long but is not pronounced very long, so no circumflex on ultima, just an acute (or grave of course) if the base form is stressed on ultima. Otherwise an acute accent on penult.