There are three major operators you can use to join keyword terms when constructing a search: AND, OR, and NOT. Each is discussed below.
Example search using "AND": Female AND Military
Using the AND operator with these search terms would retrieve results that contained BOTH terms (each result would have to contain the term "female" and the term "military").
Example search using "OR": Female OR Military
Using the OR operator would retrieve results that contained either term (each result would contain EITHER the term "female" or the term "military" - not necessarily both).
Example search using "NOT": Female NOT Military
Using the NOT operator would retrieve results that contained only the first term without including the second term (each result would only include the term "female" and could not include the term "military")
Complex search statements can be constructed by using more than one operator at a time. If you wanted to search for both major concepts, including the additional synonym keywords you thought of, you could construct a search statement that looks like this:
(female OR woman OR girl) AND (military OR soldier OR army)
That statement would look for results that had any of the terms in the first set (female, woman, girl) and then look for results that had any of the terms in the second set (military, soldier, army), then it would return results for documents that had at least one term from the first set and at least one term from the second set.
Constructing a complex statement like the one above allows you to get more results on your topic by incorporating more keyword terms that may be used to describe articles you're interested in.