1. A method comprising, by a computing device: accessing a social graph comprising a plurality of nodes and a plurality of edges connecting the nodes, each of the edges between two of the nodes representing a single degree of separation between them, the nodes comprising: a first node corresponding to a first user associated with an online social network; and a plurality of second nodes that each correspond to a concept or a second user associated with the online social network; receiving, from a client system of the first user, a first structured query comprising a natural-language string generated by a grammar model and references to one or more selected nodes from the plurality of second nodes and one or more selected edges from the plurality of edges; generating one or more query modifications for the first structured query, each query modification comprising references to one or more modifying nodes from the plurality of second nodes or one or more modifying edges from the plurality of edges; and sending, to the client system of the first user, one or more of the query modifications as suggested modifications to the first structured query for display to the first user in response to receiving the first structured query, each query modification being selectable by the first user to modify the natural-language string of the first structured query to further comprise references to the one or more modifying nodes or one or more modifying edges referenced in the selected query modification.