Final -αι (and -οι) are regarded as long in the optative ( cross169), elsewhere as short. Hence distinguish the forms of the first aorist.
|3. Sing. Opt. Act.||Infin. Act.||2. Sing. Imper. Mid.|
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].