I recommend using the ERIC Thesaurus to improve your search. The Thesaurus lists all the formal "descriptors" used by the ERIC staff. These subject words bring together articles with similar ideas, but with different terminology, under one term.
This page gives you tips on how to 1) find the best search term(s) with the Thesaurus and 2) add multiple terms to your search.
The ERIC Thesaurus is a useful tool to figure out which word(s) to use when searching to find all articles the same topic. For instance, people of a certain age might be called "young adults", "teenagers", "youth", etc. What's the best terms to find all articles on this group?
When you search the Thesaurus for "teenagers", you will see that the subject term is "adolescents". Use would use "adolescents" instead of teenagers, or other words, to bring together all articles on this age group.
The ERIC Thesaurus provides the definition of a subject word and the related, broader, and narrower subject. As you can see, "adolescents" is defined as people from ages 13-17. You can add just this term or check other related boxes to add more terms. When you click add after choosing multiple words, ERIC will add these words in a OR search that find article that have at least one of these search words.