I am a beginning user in Sketchup pro. I am now playing around with all the options. I bump into one problem with one of the faces of my model. The face is automatically divided into triangles. If i want to delete the face i have to click each triangle separately? Seems like i unfortunately added some functionality to this face. Can someone with more experience advise me how to turn my face into a normal face without the triangles?

Any three sided face can exist.
For example, you draw a triangle.
Faces with more sides can exist BUT all of the vertices must lie in the same plane.
For example, you draw a Rectangle or Circle.
BUT if you draw a shape [or move a vertex in a shape] so that ir is not all coplanar edges, then you get a ‘fold’ to form the two triangles needed to retain faces.
The edge might be ‘smoothed’ and normally unseen, but it’s still there.
If you want to see it use View > Hidden Geometry > ON toggle.
If you can erase that ‘dotted’ edge and the faces merged into one then it was all coplanar.
But if you erase the edge and the faces go too, then although it looked ‘flat’ it wasn’t.
How did you make these faces ?
Using the Rectangle tool ensures the face is one coplanar thing…

Thanks so much for your input. The View > Hidden Geometry > ON toggle did the trick. I deleted the dotted edge and repainted the plane by using a rectangular (rather than using lines). My model is ok again. Thanks