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.
Best Answer
I understand where you're coming from in wanting to use the correct phrasing with the correct people, however, from my experience, it's not so cut and dry.
Yes, you can speak informally to your friends and family, but the counterargument would be that some people talk to their friends formally, the same goes for families. Yes, you can talk to teachers and people above you formally, but there are many instances when you're close enough to these people that you can speak more casually and informally.
What I'm trying to say is: it is completely case by case. Yes, by all means, when you meet someone for the first time speak to them formally as it often conveys politeness, but you don't have to do so because they're a certain role in society.
With that said, don't walk up to the CEO of your company for the first time and utter, "Yo, what it is?" as a greeting.
Lastly, it could also depend on the environment. If that same CEO is a close friend of yours, you go fishing together and talk about anything under the sun, using the foulest of language, that doesn't mean you should do so at work, in front of other employees. In this instance, it might come off as a sign of disrespect as if you think you're above them.
Conclusion: It doesn't matter too much what role this person plays in your life, but it does matter where you are and how familiar your relationship is.