, or 出（だ）す
in its kanji form is a verb that is regularly attached to the conjunctive form
of other verbs. When it is connected in this way, it has the meaning of 'to suddenly do (A)', or 'to burst into (A)'.
To use this construction, simply add だす
to the ます stem (conjunctive form) of the verb that you want to highlight as happening 'suddenly'.
It is not good for your body if you suddenly start running fast.
After falling, I give my son candy before he bursts into tears.
, another common structure which means 'to begin', だす
quite often has the nuance of being unintentional, or something that the subject cannot avoid doing (or cannot be controlled). On the other hand, はじめる
expresses the direct intention of the subject to do (A).
At the concert, the singer will start singing. (Something that has been planned to be started)
Recently, my friend suddenly burst into song during class. (Something that happened without any prior plan/expectation)