One possibility is to search for questions tagged by a given tag sorted by relevance. This will usually1 put questions with only this tag at the top. Example:
searching for "[calculus] is:question" and choosing relevance tab
There are also various SEDE queries for this purpose (for example, this one). However, data in the Data Explorer are only updated once a week.
I will also mention that there are some feature requests on meta.SE which ask for ability to search for questions having only a given tag: