A learner asked me why we add the suffix -er to existing prepositions to form words like insider and inner. What a wonderfully curious question! I love how this person’s mind works. Talk about a teachable moment. I responded with an explanation about the versatility of -er and called attention to the parts of speech we can build using that ending.
Isn’t it both convenient and confusing when a suffix does more than one job? If you’d like to raise awareness of word parts and focus on the uses of -er, please take a look at The Suffix -er_handout. It’s geared toward lower level students.