You are searching for information on the benefits of bilingualism on children. Type your search as follows:
(bilingualism OR multilingualism) AND (children OR "young adults")
This search will return hits including at least one of the terms "bilingualism" or "multilingualism" and also one of the terms "children" or "young adults."
If searching for information on athletes, truncate it by typing in:
This will search for various forms of this word--such as athlete, athletics, athleticism, etc.
When searching for information on the benefits of green living, type the search phrase with quotation marks around it.
Typing it this way will allow search engine to look for those words together, not separate. If you didn't put quotation marks around the phrase, your search would bring back results of any information that had the word green, and/or living, and/or green living. Your results would certainly contain information that doesn't have anything to do with the topic of living in an eco-friendly way (aka green living).