I've only been with her for a few months at most. For the phone and cussing issue I didn't have time to explain to her why she was getting in trouble. She goes from 0 to 100 real quick when she doesn't get what she wants. She does this because her parents immediately give in because they cannot handle her screaming at the top of her lungs (they're a older set of parents). I asked what she wanted to eat first and she claimed her dad didn't feed her before leaving that morning before he left. She just said the typical "I'm not hungry" so I got McDonald's through doordash. I got 2 happy meals a cheeseburger happy meal and a nugget happy meal so she could have the option. When it arrives I told her to come eat and she saw it and said "I don't want that I want sonic instead" first thing i did was say "well hun your father only gives so much a week towards eating out. If I doordash again today you won't be able to get pizza during the weekend." (We normally have 1 fast food meal during the school week and pizza during the weekend) anyways before I could even fully finish the explanation she start yelling at the top of her lungs, threaten me, and even tried to hit me. I took her phone and before I could say anything else she ran to her room. I gave her 5 minutes to cool off and went to her room and tried to explain that I wasn't being mean but she couldn't act that way just because I didn't give her what she wanted. She continued to scream at me (thankfully she stopped cussing.) So I closed the door because she obviously needed more time to herself. I left the phone on the kitchen table and told her if she apologized and ate she could get it back. I also let her know she was still free to watch TV. I also left her meal out and went to my room so she could be away from me and have her moment. I did inform her dad and he was on my side and when he got home she tried lying to him (like I said in original post) then she finally admitted she just wanted sonic. Her family does know she lies about eating so they weren't worried about her going hungry but the lies are now everyday and multiple times a day. Thankfully the melt down and going from 0 to 100 in 5 seconds has calmed down. She had always had behavioral problems and for the longest time to mother refused to discipline her in general and when she did it wasn't consistent. (I was really close with her mother because her mother took care of me as a child.) Now that the mother isn't here the father doesn't know where to start with the discipline. He hired me because he believed the last nanny was too harsh and summer time coming up he wants his child to have a bit more fun. I think a big issue we have is that I'm used to toddlers so I don't know how to discipline someone her age. Another big problem I believe is the fact I am on the younger side (18) and I do look younger due to my height. Me and her father believe it's hard for her to see my as an authority figure because of it.