Some databases have a thesaurus with preferred terms called subject terms. Sometimes you can get a better search result if you use subject terms than keywords of your choice. See the following video: Using subject terms in MLA International Bibliography.
If it is not possible to search on subject terms or if you get too few results, you can try searching on keywords of your choice. Try searching on the following types of words:
Synonyms (e.g. old people / elderly / senior citizens)
Narrower terms (e.g. working class, lower class, middle class and upper class are narrower terms than social class)
Words with alternative spelling (e.g. theatre / theater ; loanwords / loan words)
Variants (e.g. Canadian schools / Canada's schools / schools in Canada)
Sometimes it can be good to search on words with the same meaning in different languages. For example, if you are studying French, you could try searching on both French and English terms.
Sometimes it can also be good to search on related terms (e.g. "slaves" is related to "slavery" and "women's rights" is related to "feminism")
Say for example that you want to find literature about puns in advertisements. The search could then for example look like this:
(puns OR wordplay OR "word play" OR "play on words") AND (advertisements OR commercials)