Unlike Google, library databases can't understand an entire sentence. So you'll need to break your topic down into the most important ideas - the KEYWORDS.
Example Topic: How do Maria Montessori's theories contribute to the early childhood education/child development?
The specifics of your topic will matter when selecting sources, but for searching you only need the most essential components.
Keywords: Montessori, early childhood education, child development theories, childhood development
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!
Example:
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.
Library databases have built in search tools. Try some of these:
Look on the left of your search results, or for an "advanced search" page to find these tools - and more!
Use the operator AND to find only sources that mention both keywords.
This search will bring back fewer results than searching either keyword on its own.
Use the OR operator to expand your search with additional keywords.
Use the “QUOTES” strategy to search for several words in a phrase. 
This will bring back results that only use that exact phrase.