Some more practice with nominative and accusative forms and usage.

ὁ ἄνθρωπος ἰσχυρός ἐστιν.
βλέπω τὸν ἰσχυρὸν ἄνθρωπον καὶ φεύγω.
ἡ κόρη καλή ἐστιν.
βλέπω τὴν καλὴν κόρην. οὐ φεύγω.
Accusative singular   masculine ς becomes ν
femine (  α  or  η  ) adds that  ν  

οἱ φίλοι τε καὶ αἱ φίλαι δειπνοῦσιν.
nominative plural    -οι for masculine
-αι for feminine

τοὺς δούλους καλῶ. καλῶ καὶ τὰς δούλᾱς. female slaves of course
accusative plural    -ους for masculine
-ᾱς for feminine
Both plural accusative endings are longish, that's why there can only be an acute stress on the penult, no circumflex. See accents if you want to brush up on your knowledge of stress and accentuation.

Some more practice for YOU

Visualize the following and say what they are, what they're like (nominative)
or what you see, hear, feel, eat etc (accusative).
ἡ πέτρα (rock, petrified, Saint Peter was supposed to be rock solid)
ἡ γλῶττα (language and tongue: boy sticking out tongue, tongue in butcher's window, on your plate, doctor looking at it)
ἡ θάλαττα (the sea, very important for Greece: fishing, sea-faring)
ἡ μάχαιρα (knife, Philip will use his grandfather's knife to kill a wolf in lesson 5b)
ἡ κρήνη
ἡ οἰκία

Example:
Imagine a ὑδρίαν, a very μεγάλην ὑδρίαν. The ὑδρία is very καλή. You would like such a καλὴν ὑδρίαν. The ὑδρία must be rather χαλεπή (troublesome, hard on one's head). To carry such a χαλεπὴν ὑδρίαν on one's head δυνατόν ἐστιν but certainly quite χαλεπόν. And a very ἀργὸς δοῦλος would most certainly say: οὐ δυνατόν ἐστι τὴν χαλεπὴν ὑδρίαν φέρειν. (I'd say: this blimmin jar!)

Mary is a κόρη, a καλὴ κόρη, Tess is a μάλα καλὴ κόρη, ἀλλὰ καὶ χαλεπὴ (poor boys, she orders them around!) but Monica οὐκ ἔστι καλή, poor thing, and on top of it all she ἀργός ἐστιν. Boys like καλὰς κόρας but teachers don't care, what they don't like is ἀργοὺς κόρας. And so on and so forth.
 
Keep in mind that we have to say:
αἱ κόραι καλαί τε καὶ ἄοκνοι εἰσίν.
but some κόραι ἀργοί εἰσιν.
I like τὰς κόρας τὰς καλάς τε καὶ ἀόκνους.
I scold τὰς κόρας τὰς ἀργούς.

Compound adjectives don't have a special feminine form, for them masculine and feminine are identical.