Most words have synonyms that mean the same, or very similar, things. For each keyword in your topic, try to come up with at least one synonym. Not all keywords will have synonyms, but many do!
Keep an Eye Out
Sometimes scholars use terms that you might not be familiar with, or which might mean something very specific within the discipline. While searching, look for unfamiliar terms or words that show up a lot. Try searching for those and see if you find more relevant sources.
Look on the left and right of your search results, or for an "advanced search" page to find these tools - and more!
You can evaluate any source using the 5 W's: