“If” is used frequently in casual speech and writing where some others would prefer “whether”: “I wonder if you would be willing to dress up as a giant turnip for the parade?” Revise to “I wonder whether. . . .” “If” can’t really be called an error, but when you are discussing two alternative possibilities, “whether” sounds more polished. (The two possibilities in this example are: 1) you would be willing or 2) you wouldn’t. In sentences using “whether” “or not” is often understood.) Don’t substitute the very different word “whither,” which means “where.”