Good morning everyone. I am recently doing a Blender course on youtube. Here is the link of the youtube video: https://www.youtube.com/watch?v=LRwWj_FFoM4. Just until I almost finished my plane, I realized that the peak was totally deformed, and now I can no find a way to fix it. Here is the link of the airplane img: https://postimg.org/image/euj6314ln/
Please help me! thank you 
If you have an error then just select the vertices that cause the error and press X, then select delete.
Then go back and follow the tutorial properly.
Don’t be afraid to just delete everything you have made and start again from scratch, it’s easier and quicker than trying to fix a crap model.
Also ensure you are in orthographic view when the tutorial is in orthographic view and perspective view when the tutorial is in perspective view. Toggle between the two with numpad 5. You will see which view type is being used in the top left corner of the 3d view.
It looks to me like you need to turn down the merge distance on the mirror modifier.