想要更好的掌握你所学习的外语,去了解当地人所遵行的日常习俗和礼节
这个句子可看成Itisagreathelp4theperson后面的whoislearningaforeignlanguage修饰person,2knowsomecustomsandmanners4thepeoplewhospeakit是实际主语,itis是形式主语,4thepeoplewhospeakit是修饰2knowsomecustomsandmanners.
整个句子可简写成2knowsomecustomsandmanners4isagreathelp.