I think this might be a duplicate, but I will continue on anyway.
Just as the question says, when would it be inappropriate/out of place to speak formal English? The reason I ask this is because I am still learning formal and informal language. Also, why would it be inappropriate to speak formally during that time? I read that informal speech is used with friends and family, but is formal speech different?
According to http://www.phrasemix.com/concepts/casual-speech: Use casual English with your family and friends. If you speak formally with these people, it may seem like you're angry with them or trying to push them away.
Oh, I also apologize for asking this, I am not a native speaker of English.