N4
だす
To suddenly begin, To start doing
転んだ息子が泣きだす前に、あめをあげる。
I give my son candy before he bursts into tears after falling.
Explanation
「だす」 after the stem of a verb means 'suddenly start …': it comes without warning and with force.
「急に泣きだした」 (He suddenly burst into tears). Typical for rain, snow, crying, laughing or breaking into a run.
Mixed up with
「~だす」 is a sudden outburst; 「~はじめる」 is just starting from a beginning, and works for natural events and planned things alike.
Not for planned things: not 「×明日から勉強しだす」 but 「明日から勉強し始める」.
Crying, laughing, sudden running and showers almost always use 「~だす」.