I need HELP!

i’m trying to do this:

but it does nothing,

Try this, select those 4 verts, and hit the F Key. That does the same thing. If that doesn’t work that means that the one vertex closest to the grey part is bot letting you, so all you have to do is just move that just barely and try again. The first time you tried it was trying to make a concave quad, I think that is what it is called, but they don’t allow that. You have to make sure ,at best, those three verts make a straight line, but can’t go in anymore. Hope that helps. It is kinda hard to explain,

thanks heaps it worked