We add the endings -er and -est to words when we want to compare things.
The purple car is old, but the orange car is older and the black car is the oldest.
Often we just add the -er and -est to the word but sometimes we have to make changes to the word before we add the ending.
If the word ends in 'e' we just add -r or -st.
nice / nicer / nicest
If the word ends in 'y' then we drop the 'y' and add -ier or -iest.
happy / happier / happiest