Great question!
Databases are great at storing and providing access to expensive published materials not freely available, but they are not great at parsing out a research question like Google or other open web search engines are. Databases are only able to match exactly what you typed in with the exact words that appear in the databases' items. This means that if you search for a misspelled word, the database probably won't find anything unless one of its items also misspelled that word. It also means that including words like what, why, and how in your search will confuse the database because so many sources have those words. Because of this, you have to just use specific keywords for your topic or from your research question and then join them together with the word AND. For example, for the research question: How does pollution affect asthma in children? you would want to search for "pollution AND asthma AND children."
Click here to view a video tutorial on creating a search statement