A-stems: the simplest way to sort this out is to learn each type as a list:

I.  The important group is the long-A stems  

long-alpha (after e, i, r	singular endings	plural endings (same)
ἡ ἀγορ-ά  (marketplace) τῆς ἀγορ-ᾶς ἡ οἰκί-α     (house) τῆς οἰκί-ας ἡ ὑδρί-α     (water jar) τῆς ὑδρί-ας	--α  (nominative) --ας (genitive --ᾳ   (dative) -- αν  (accusative)	---αι --ῶν --αις --ας
eta
ἡ ἑορτ-ή (festival) τῆς ἑορτ-ῆς ἡ κόρ-η (girl) τῆς κόρης ἡ κρήν-η (fountain) τῆς κρήν-ης	--η  (nominative) --ης (genitive --ῃ   (dative) -- ην  (accusative)	---αι --ῶν --αις --ας

Practice with basic phrases, adding familiar adjectives(ἰσχυρός, -ά...μακρός, -α ...μικρός, -α...  πολλοί, πολλαί... χαλεπός, -η, etc.)

for verb subject use nominative, with prepositions use proper case

(out of/ from = genitive; in/at = dative; toward = accusative).

Remember adjectives follow the same rule:  after e, i, or r, long-alpha stays:

so ‘in the small house’ --> ἐν τῇ μικρᾷ οἰκίᾳ

toward the small house

out of the dear fountain

into (εἰς) the long waterjar

the small girl carries

(with)in the hard marketplace

at the many fountains

the strong girls

(they) bring big waterjugs

Now change all the phrases (above) singular to plural or vice versa.

[[click here for key]]

II. The other group of feminines are short-a stems. Only two of any importance for us:

ἡ θάλαττ-α ἡ μέλιττ-α

-α -ης -ῃ -αν

-αι -ῶν -αις -ας

III. Also exceptional--but just as important-- are the masculine ᾱ-stems:

notice, these follow the same rule, ᾱ --> η except after e, i, & r;

and (like the short-a fems.) these 'borrow' the genitive singular

ὁ δεσπότ-η-ς ὁ νεανί-α-ς (young-man)

-ας/-ης -ου -ᾳ/-ῃ -αν /-ην

-αι -ῶν -αις -ας

Now practice with types II & III: (with adjectives from ch. 4)

into the great sea

for the hard master

of the strong young-men

of the fair Melissa ('honey-bee')

we love the easy master

but not the hard seas (DO)

And change singular to plural and vice versa:

[[ Click here for key ]]