Hi - Im a complete beginner - have followed the threads of many with the same issue but none of the solutions are working for me. Can somebody please tell me why I am unable to cut out the centre for doors and windows by pushing. I have attached a simple version of my approach for file size and your feedback is gratefully received. trial for windows.skp (2.3 MB)
Your walls are inside a group container and your rectangles for the openings are outside. You need to open the group for editing first. Then draw the rectangles and push them through. Make sure you stop at the inside face, too. I double clicked on the wall with the Select tool to open the group for editing. You can right click on it and choose Edit Group, too.
Thank you so much @DaveR this is the best example I have seen and so obvious. I am off to chop out some windows. 
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You may want to view the tutorial SketchUp Fundamentals
Thank you