Here's a breakdown:
* -er: This ending is used for most one-syllable adjectives and some two-syllable adjectives ending in -y.
* Examples:
* big - bigger
* fast - faster
* happy - happier
* -ier: This ending is used for two-syllable adjectives ending in -y.
* Examples:
* easy - easier
* busy - busier
* pretty - prettier
However, there are some exceptions:
* Irregular comparatives: Some adjectives have irregular comparative forms that don't follow the -er or -ier rules.
* Examples:
* good - better
* bad - worse
* far - further (or farther for distance)
* More/Less: For longer adjectives (especially those with three or more syllables), we use "more" or "less" before the adjective.
* Examples:
* beautiful - more beautiful
* difficult - more difficult
* interesting - less interesting
So, while -er and -ier are the most common endings for comparative adjectives, remember to check for irregular forms and use "more" or "less" when appropriate.