3rd Declension Much Simplified:

 

The defining feature of the 3rd declension (as opposed to the first 2) is that here we have consonant-stems (mostly). So the endings are added to that stem differently (basically, after a consonant you need a vowel)

 

 

With these nouns (even more than the others) you need to learn nominative and genitive (with article to tell gender).

 

So for παῖς (either or ) we learn from the genitive, παιδός that

the stem is  παιδ-      to which we add

the endings,         -ος  (gen.)  --> παιδὸς

          (dat.)                   -ι       ---> παιδὶ

          (acc.)                   - α   ---> παῖδα

 

the plurals are: nom.  –ες  --> παῖδες

                   gen. -ων   ---> παίδων

                   dat. -σι    ---> παισί

                   acc.  -ας    ---> παῖδας

 

Notice that dative and accusative plurals just add –s- to the case-markers.

But in dative plural the –s- and –i- reverse order, and –d- + -s- simplify,

as in the nominative singular παιδ + ς --> παῖς