Commencer à faire and se mettre à faire both express the beginning of an action. The two expressions are close in meaning, but se mettre à often emphasizes more the moment when the action starts.
Commencer à + infinitive
Commencer à simply indicates the start of an action.
- Je commence à comprendre. (I am starting to understand.)
- Il commence à pleuvoir. (It is starting to rain.)
- Nous commençons à travailler. (We are starting to work.)
Se mettre à + infinitive
Se mettre à also indicates the beginning of an action, often with the idea of a more visible or sudden change.
- Il se met à rire. (He starts to laugh.)
- Elle se met à chanter. (She starts to sing.)
- Je me suis mis à pleurer. (I started to cry.)
Compare
- Je commence à apprendre le japonais. → simple beginning
- Je me mets à apprendre le japonais. → emphasis on starting the action
- Il commence à neiger. → beginning of snow
- Il se met à neiger. → the snow starts suddenly
Simple difference
- Commencer à = start of an action, in a neutral way
- Se mettre à = start of an action, with a stronger idea of change
In many sentences, both are possible. The difference is often small.
Advice
Remember this simple idea: commencer à is more neutral, and se mettre à highlights more the moment when the action begins.