Before you begin a keyword search, think about your topic. Decide on some words (terms) to express the most important concepts.
Next, think of additional terms which might express the same concepts.
If you do this work before you begin your search, you'll be ready to create many different searches using the list you prepare.
Search engines are getting "smarter", so it seems like they're doing the thinking for you. They often automatically search for alternate terms for you. This is a great time-saver.
But don't be fooled. They're not really smarter, because they don't think. Really they're more "sophisticated". The designers have built in a system for identifying synonyms. It's still helpful for you to think about your search terms, especially when your results are too broad or too few for your needs.
For an example, let's use the topic "Are pregnancy rates rising among teenage girls in Canada?".
Now we need to think of other ways of expressing our terms. Any of the terms in the chart below would be effective.