17. The method of claim 1 , wherein generating the one or more query modifications for the first structured query comprises: accessing a context-free grammar model comprising a plurality of grammars, each grammar comprising one or more query tokens; identifying one or more grammars, each identified grammar having query tokens corresponding to each of the objects referenced in the first structured query and at least one additional query token or alternate query token; and generating one or more query modifications corresponding to one or more of the additional query tokens or one or more of the alternate query tokens from the identified grammars.
1. A method comprising, by a computing device: receiving, from a client system of a first user of an online social network, a first structured query comprising a natural-language string generated by a grammar model and references to one or more objects associated with the online social network, wherein the grammar model is a context-free grammar model comprising a plurality of grammars, each grammar comprising one or more non-terminal tokens and one or more query tokens, each grammar being an ordered sub-tree adjoining one or more other grammars via a non-terminal token; generating, responsive to receiving the first structured query, one or more query modifications for the first structured query, each query modification comprising references to one or more modifying objects associated with the online social network; and sending, to the client system of the first user for display to the first user, one or more suggested modifications for the first structured query corresponding to one or more of the query modifications, respectively, each suggested 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 of the modifying objects referenced in the query modification corresponding to the selected suggested modification.