Keep it simple and use descriptive keywords as search terms. They should be relevant to the topic that you are searching.
“Alice in wonderland” filetype:pdf: it finds specific document types, such as pdf, ppt, doc, odt, etc.
intitle: “free ebooks”: it restricts your search to the titles of web pages.
linux distributions -ubuntu: it finds linux distributions which are not Ubuntu.
define word: it finds word definitions and synonyms.
translate “me gusta el arroz” to English: it translates words and sentences instantly
“We the People”: it searches for an exact word or phrase.
1 Euro to dollar: currency converter.
5 inches to centimetres: unit converter, [number] [unit] into [unit].
25% of 43, sin(1/x), log(4)*sqrt(3): Google Search has a built-in calculator. It can also plot mathematical functions.
London weather, London time: forecast of the weather, current time in any given city.
Capital of Germany, longest river in Africa, Mozart’s death: simple factual information.
Use SafeSearch to filter explicit results: Search settings, Filter explicit results.
selfhelp site:justtothepoint.com: search within a site.
DuckDuckGo is not safe to browse as Microsoft tracks user data, and yet it is one of the most privacy-focused search engines on the market.