Pawn Endgames constitue a basis of all endings
One should study them most carefully, because each endgme can eventually transpose to pawn endgame. Despite their simplicity, Pawn endigns are very complicated and even masters oftem err on them...
Rule of Square
Outside Passed pawn
Corresponding squares, Triangulation
Use of Stalemate in Pawn Engams
There are many other subjects on pawn endgames such as breakthrough, Extra Space, Transition from Pawn to other Endgame, Transition from other endgame to pawn endgame etc.