I'm absolute beginner, trying to figure out how css works. I made an experiment modifying a template, which runs great so far.
But I tried to put a drop-down menu and the result was a complete disaster - the menu itself doesn't look as it should, the other menu on the site absolutely changed its appearance and besides when the menu "drops down" it pushes down the areas of the site which are below it ... Please, help me find the mistakes and put a working drop down menu in the place where I want it to be.

I have uploaded the files of the site here:

and this is the original of the drop down menu:

Please, let me know what I'm doing wrong and how to fix it! Thank you!