You can use quotes to combine words together:
ex. "World war II"
You can use an * or an ? to represent a wildcard. The * can represent 0 or many characters. The ? can represent 1 single character.
ex. econom* will find both economics as well as economies.
You can use AND, OR, NOT in between words or phrases to combine them with boolean logic.
ex. (china OR india) AND economy will find records that deal with both china economy or india economy.