We add the endings -er and -est to words when we want to compare things.
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