My brother is a grammar Nazi about my text messages.
grammar Nazi
idiomA person who habitually criticizes the grammar and (often) usage in others' speech or writing, often in ways that are sometimes not even linguistically accurate.
Origin
first recorded 20th c.First use appears c. 1991. From grammar + Nazi (“one who imposes one's views on others; one who is considered unfairly oppressive or needlessly strict”).
She acts like a total grammar Nazi whenever someone uses the wrong form of their or there.
Although he considers himself a helpful editor, his friends often find his behavior as a grammar Nazi quite exhausting during casual conversation.