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 prevalence of dengue in South America and ways to prevent it from spreading. 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 rearch question/topic. Try to think of words that others might have used, and then combine them using Boolean opeartors (AND, OR and NOT).
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
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 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.