複合動詞N4
〜だす
to suddenly start doing...
Structure
V-ます形（語幹）+ だす
Explanation
Attached to the masu-stem of verbs, 〜だす expresses an action suddenly beginning, "to suddenly start doing..." (e.g., 泣きだす, 雨が降りだす).
Examples
赤ちゃんが急に泣きだしました。
The baby suddenly started crying.
空が暗くなって、雨が降りだしました。
The sky darkened and it started to rain.
Note
〜だす emphasizes a sudden start. Unlike 〜はじめる, which has no sense of suddenness.
Related Grammar
〜つづける〜はじめる