The theme of control is evident in 'Emma' implicitly and explicitly. Austen's novels take place in a strictly regimented society, where social position impacts on the opportunities and level of freedom faced by the characters.
In 'Emma' we see a variety of examples of control at work. We also see numerous examples of lack of self-control (for example Emma's rudeness to Miss Bates, Frank Churchill's frivolous behaviour).
Emma exerts control over Harriet; under the guise of friendship she persuades Harriet to reject Robert Martin's proposal and set her sights higher.
Harriet's expectations in marriage are controlled by social convention as she is:
"the natural daughter of somebody"
The doubt surrounding her parentage means that Mr Elton is horrified at Emma's suggestion that he should marry her. This also demonstrates Emma's naivety.