〜始める, 〜続ける, and 〜終わる: stages of an action
Attach these verbs to a masu-stem to express the beginning, continuation, or completion of an action. Distinguish the compound forms from independent verbs and follow the conjugation of the final element.
Mark the point when an action begins
Remove ます from a verb such as 降ります and add 始める, 続ける, or 終わる, as in 降り始める. The final element carries the conjugation of the whole expression.
空が急に暗くなり、雨が降り始めました。 marks the onset of the rain. Its focus differs from 降っています, which simply presents the rain as already in progress.
Follow a continuing process to its end
彼女は三時間、黙って絵を描き続けました。 means that she kept drawing silently for three hours. 続ける focuses on the action continuing over a span of time.
この章を読み終わったら、内容を短くまとめてください。 asks someone to summarize the chapter after finishing it. 終わる presents the preceding action as reaching completion.
Separate independent verbs and stopping midway
授業が始まる says that the class itself begins, while 先生が話し始める says that the teacher begins the action of speaking. Check whether a masu-stem precedes 始める to identify the compound.
途中で読むのをやめました。 says that someone stopped reading partway through, so it differs from 読み終わりました, which reaches the end. Do not treat stopping and completing as the same event.
Quick check: To say that a student is still running, which fits: 走り始めています, 走り続けています, or 走り終わっています?
Show the answer and why
走り続けています fits. It focuses on the running continuing from its beginning through the present.