Verb- mis à pleurer

Frances, it is better to give the full example you are referring to, or at least a link.

However, are you it wasn’t the verb expression ‘ se mettre à faire quelque chose ( pleurer ) ‘, which is a fixed expression that can be translated in English as ‘ start to (do something) ‘.  

Why? Because that is how the expression is used in French. Grammar and expressions often are best learned as they are, rather than trying to deconstruct them.

 https://www.wordreference.com/fren/mettre