A couple small exceptions to the “add -er” rule:
If the stem ends in -e, only add -r:
liese
quiet
→lieser
quieter
If the stem ends in -el or -er drop the -e from the stem ending before adding -er:
flexibel
flexible
→flexibler
more flexible
teuer
expensive
→teurer
more expensive
Some adjectives and adverbs have irregular comparative forms. We’ll go through these further down.