In your example, those aren't spelling errors, those are grammatical errors. Creating random spelling errors is fairly straight forward, creating grammatical errors is quite a bit more challenging.
I'd recommend finding a word list with part of speech information, that includes all case-number-gender declensions (he, him, his, etc). Then, when you find a word that has multiple declensions, switch it with one of the others randomly.
If you want to be particular about swapping only verbs, or only nouns, you're pretty much not going to do it reliably, because writing a computer program to reliably tell when a word is being used as a verb, noun, or adjective is a significant challenge in and of itself.