Ungrammatical describes language that breaks the rules of proper grammar. When a sentence is ungrammatical, it's constructed in a way that sounds wrong or confusing to people who speak that language fluently.

For example, “Me and him went to the store” is ungrammatical in standard English because the more accepted form is “He and I went to the store.” The sentence “She don't like pizza” is ungrammatical because “don't” should be “doesn't” when talking about one person. These mistakes might not completely block understanding, but they signal that the speaker isn't following standard grammar rules.

Sometimes ungrammatical sentences are hard to understand: “Book the read I yesterday” makes little sense because English word order is scrambled. Other times, like with “I seen that movie,” the meaning is clear even though the grammar is wrong (it should be “I saw” or “I have seen it”).

Native speakers usually recognize ungrammatical sentences immediately because they sound off, even if they can't explain exactly which rule was broken. Using standard grammar helps you avoid ungrammatical writing and speaking, which matters especially in school, job applications, and other situations where people may judge your education and attention to detail.