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There are plenty of ways to find that trope you're looking for.
Use our inbuilt search engine. It's under Tools, click on "Title Search".
However, the on-site search function looks for the exact words you type in (with and without spaces), and some of our titles are rather misleading, metaphorical, or allusive, so it may not be the best way to find a page you're looking for.
Look up a series, book, or story that contains a very clear example of the trope you have in mind, then skim the "Tropes" list or the list of pages that link to it (accessible via the "related to" button at the top of the page.)
Use the Google custom search bar, located at the top of the page.
You can also add "site:tvtropes.org" to the phrase you're searching for in regular Google, to limit your search to the wiki.
If you're lucky, there'll be a smaller index that your trope very obviously ought to belong to if it already exists (like, say, Title Tropes). If so, read through that. Index-Index is a list of all existing indexes.