Common Writing Mistakes

I ensure you the product are of highest quality.

ensure係確保

當你想講向....保證, 要用assure 

I want you to send me the files right now.

Grammar 係冇錯, 但係如果係工作email就好冇禮貌, 你可以用Could you....?/Please....

Our headquarter is located in Central.

當講緊總部 (noun), 係headquarters, 冇左"s"就會變左verb, 意思就會變左 將總部設在..., 所以正確係"headquarters"

Our staffs have been trained for over a year.
staff本身已經係plural, 唔可以加"s"