Library databases, and other search engines, are often very literal. When you type words into a search box in a library database, the database searches for those exact words. To make your search more effective, pick the major concepts or most important words from your research topic/question, rather than a complete sentence.
Once you have your major concepts selected, think of synonyms or related terms for these major concepts. Google, Wikipedia, dictionaries and thesauri can help with this.
For example, let's say you're researching the legal and ethical aspects of physician-assisted suicide. Some possible major concepts include:
For each of these major concepts or keywords, I can try to think of synonyms or related terms to use in my search. Authors don't always use the exact words you might have in your research question/topic. Try to think of words that others might have used, and then combine them using Boolean operators (AND, OR and NOT).
Truncation tells the database that you will accept other forms of a word. It will expand your search results. To use truncation, cut off the end of a word and replace it with an asterisk (*).
medic* → gets you medic, medical, medicine, medications, medicate...
insur* → gets you insurance, insured, insuring...
Sometimes searching just for particular words isn't specific enough and will return many unrelated results. If you only are interested in particular words found in a certain order, you can put quotation marks around them to search them as a phrase. If you are searching in PubMed, Medline or CINAHL, though, it is better to not use quotation marks. They have a feature that aids in searching and is turned off when you use quotation marks.
I'm interested in finding information on quality assurance in healthcare. Using "quality assurance" will return articles that have those two words next to each other. If I just put in quality assurance without the quotes, it will return any article that has quality or assurance in it.