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.