I knew this question was there: When is it appropriate to use ‘that’ as opposed to ‘which’?, but I tried searching for it and couldn't find it. Many usage questions concern function words in English, so it would be nice if we could search for those function words and get reasonable results.
Additionally, the search page itself has a Google search option
To get there enter enter in the search box (eg leave the search terms blank). This page is also linked in the sidebar on every search result page under "search tips".
Unfortunately, the current Stack Exchange search tool filters out stop words prior to indexing the site. The site's search tools are better suited for finding combinations of tags and indexed text. For pure text searches, it is better to use Google Search using the
site:english.stackexchange.com <your search query>
A Better Search Option
I find the "Related Questions" search extremely helpful for finding questions. Click on the
Ask Question link and type in the title as clearly as you can remember it. The "Related Questions" will often do a much better job at finding the question than the native search tool.
Search now does a much better job of this, which is no longer a stop word - take a peek: https://english.stackexchange.com/search?q=appropriate+use+of+that+and+which