"Think about the sheep when you cannot sleep."
Translation:Nghĩ về những con cừu khi bạn không thể ngủ.
hãy is placed before the verb to make the sentence a strong command (imperative) hãy đi ngủ = go to sleep! (the mother ordering the children to go to sleep)
đi placed after the verb is an informal suggestion đi ngủ đi = go to sleep (husband telling the wife she is too tired and should get some rest)