Participating in pre-training assessment helps you learn about which modules you should focus on. 参与训前评估有助于您了解自己应当主要学习哪些模块。

句子成分分析：动名词短语Participating in pre-training assessment作主语；helps为谓语动词，you为helps的宾语，省略to的不定式短语learn about which modules you should focus on用作宾语补足语。同时，在这个宾语补足语中，又包含一个由which引导的宾语从句，用作介词about的宾语。