What is the function of し here?
日本語を勉強し始めたきっかけ、そして日本語学習の目標は何ですか
How does it differ from
日本語を勉強始めたきっかけ、そして日本語学習の目標は何ですか
This し is the masu-stem (or continuative-form) of する. masu-stem + 始める
means "to start V-ing". For example, 食べ始める means "to start eating", 見始める means "to start watching", and 運転し始める means "to start driving".
日本語を勉強始めたきっかけ is simply ungrammatical because 勉強 is not marked with a particle, and を cannot be an omitted particle because を is already used before 勉強 (日本語を勉強をする is wrong).
So here's the summary:
without 始める | with 始める | |
---|---|---|
勉強 as a suru-verb | 日本語を勉強する to study Japanese |
日本語を勉強し始める to start studying Japanese |
勉強 as a noun | 日本語の勉強をする to do study of Japanese |
日本語の勉強を始める to start study of Japanese |