In my previous post "Gmail Shortcut Key", I wrote about Gmail shortcut key "?" using which you can easily access Gmail mostly features without moving your cursor around Gmail window in your browser. In Yahoo! mail, this feature also exist. You don't need to remember about mostly shortcut keys like 'N for compose' , ' I for start instant message', as by pressing only "?" without quotes from your keyboard, you can easily access all features, just look for the important commands. So don't waste your time for searching about Ymail shortcut keys as it has inbuilt feature where you can look for any general shortcut key.
So just press ? using shift key, a pop-up window will come with all shortcut keys.