Disabling stem expansion. Entering a capitalized word in any search field will prevent stem expansion (no search for gardening if you enter Garden instead of garden). This is the only case where character case should make a difference for a Recoll search.
Phrases. A phrase can be looked for by enclosing it in double quotes. Example: "user manual" will look only for occurrences of user immediately followed by manual. You can use the This exact phrase field of the advanced search dialog to the same effect.
Query explanation. You can get an exact description of what the query looked for, including stem expansion, and boolean operators used, by clicking on the result list header.
Quitting. Entering ^Q almost anywhere will close the application.
Closing previews. Entering ^W in a preview tab will close it (and, for the last tab, close the preview window).