Select the most important 2-3 terms related to your topic. Put the word AND between each new term; this tells the database to look for resources with ALL your terms.
korea AND diplomacy
Use quotation marks ( " " ) around phrases (two or more words); this tells the database to look for the words in that specific order.
"ming dynasty" AND korea AND diplomacy
Use an asterisk ( * ) to find multiple endings of one word. Using diplo* as a search term will include results that refer to diplomat, diplomacy, diplomatic.