How can i create a trianle face ?

[Stem]

To connect vertex across edges, use "Connect"

quick example:-

Simple cube with multiple edge loops. RMB(for menu)-> open tools menu:-



select "Connect":-



move mouse over vertex or edge for first selection, when highlighted, LMB to select



move mouse to next selection(edge or vertex on same model), a line will drag out showing where it will be created:-



LMB on selected vertex or edge to create connection:-



RMB to exit the connect tool.

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To add the triangle face, you need to create it as a solid. If you are adding the triangle to that model, so it is part of the model(not a separate part), you can make edit, but you need to be careful, as it can be easy to make a mess and corrupt the model.

[please note, I am no expert with wings, this is just the way I would create the geometry)

Quick example:-

model with 3 vertex selected showing where I want the triangle:-



first create an edge loop(shown in red)



select the corner edge and delete it:-



With the 2 vertex (shown) selected, press "C" to connect them



there are also the 2 other vertex behind the ones you have already connected that need selecting and connecting, so that it is then a solid.



Hope you can follow that OK.