Hints & Help for editors
Adding a form
To add a spreadsheet for, only if you get the error that the "The URL you entered is not supported" :
Make you form as normal, save it and return to Google Docs
Open the spreadsheet made by the form and use the Form menu and then Embed
Copy the embed code (which includes the <iframe> tag) to a text editor
From the embed code copy the URL in the src="" attribute
Go back to your site, go to embed the form and use the URL got from above
Copying a page from another site
If you select a web page in your browser (C-a C-c for example), and then past it on Google Site new page form, you'll later have a LOT of problems (first one : the fon problem). So, most of the time you just paste the content to a dumb editor (C-v) then put it again in the cut buffet (C-a C-c), and then paste it inside Google Sites. All the formating is gone, time to do your own (eg. re do the list, insert pictures, etc).
Adding a link inside a web page
The easier way is to do as described in Google help : create a table of contents and use the links generated there.
Remember : Using Google tools = the art of finding workarounds.