Declension table

puella, puellae (f) - girl
Case sing. pl.
Nom. puella puellae
Acc. puellam puellas
Gen. puellae puellarum
Dat. puellae puellis
Abl. puellā
Notice that the ablative singular is spelt with a long -a > puellā. This will not always be shown in texts.
puellis

Notice that the:

  1. genitive singular
  2. dative singular
  3. nominative plural
all have the same endings.

The nominative is the subject of the verb. If you see this ending, therefore, check the verb. If it is plural, then there is a high chance that the -ae ending is the nominative plural. If it is singular, then the -ae ending has to be singular.