Why does the opening of an epoxide occur via an Sn2 like mechanism when using a methanoate ion as a nucleophile and methanol as a solvent?
Quotation from Clayden: 


  Without protonation, the epoxide oxygen is a poor leaving group, and leaves only if ‘pushed’
  by a strong nucleophile: the reaction becomes pure SN2. Steric hindrance becomes the controlling
  factor and methoxide attacks only the primary end of the epoxide.