- Когда не можешь кончить, не начинай
- Never undertake a job if you are not sure that you will be able to accomplish it
Cf: Better never begin than never make an end (Am.). Better never to begin than never to make an end (Br.). Don't start anything /what/ you can't finish (Am.)
Русско-английский словарь пословиц и поговорок. Академик.ру. 2013.