Epoxides experience a great deal of ring strain and as such are rather reactive. They undergo ring-opening reactions under acidic or basic/nucleophilic conditions.
In nucleophilic ring-opening the epoxide is initially attacked by the nucleophile. Epoxides are unstable because of ring strain and are rather good electrophiles. A variety of nucleophiles including -OH, amines, Grignard reagents, NaCN, and alkoxides (NaOR).
For example, treating ethylene oxide with NaOH followed by an acidic water quench results in ethylene glycol.
The nucleophile always attacks the least substituted carbon (SN2 like). Typically a quench is required to protonate the alkoxide as shown in the second step above.
Nitriles are prepared by treating epoxides with NaCN.
Ethers are prepared if alkoxides are utilized.