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 I am researching nutritional strategies to minimize fatigue in endurance athletes. The major concepts for this search would be:
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 rearch question/topic. Try to think of words that others might have used, and then combine them using Boolean opeartors (AND, OR and NOT).
Use AND to narrow your search.
When using AND, it will find records that have both terms in them. For example the following will search for articles that have both diabetes and hypertension somewhere in the text when doing a keyword search.
Use NOT to narrow your search.
When using NOT in a keyword search, it will exclude records that have a related term you don't want to include or a term that could be implied by your other search terms. For example the following search on weight loss will exclude any articles about anorexia.
Use OR to expand your search.
When using OR in a keyword search, it will find records that have either or both terms in them. For example the following will search for articles that have either high blood pressure or hypertension or both.
A wildcard tells the database that you will accept other forms of a word. It will expand your search results. To use a wildcard, cut off the end of a word and replace it with an asterisk (*).
bio* → gets you biology, biological, biologist, biomedical, bioscience, biotechnology, etc.
cancer* → gets you cancer, cancerous, cancers
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 out the effects of river pollution on human health. Using "physical activity" will return articles that have those two words next to each other. If I just put in river pollution without the quotes, it will return any article that has physical or activity in it.