這英文文法有沒有問題??,

首先, If I were you, 是 I would... 而不是 I will...

至於 go on a diet, 字典的解釋是 to eat only certain foods or a small amount of food because you want to lose weight. 即已包含 to loss weight 的意思. 所以不用重覆.

全句是: If I were you, I would go on a diet.

文法冇問題, 但句子有啲奇怪..

因為前面 'If I were you' 感覺好casual, 但 ' achieve weight loss results' 好formal 同累聚..'weight loss' 已經係一個team, 唔需要加'result'

你可以試用:
If I were you, I will go on a diet to lose some weight.

在002 的答案，文法是正確的。
但簡單一點，你可以說：

If you were you, I would go on diet to lose some weight.

If I were you, I would go on a diet with a view to losing some weight.

If conditional 既呢個類型個 main clause (comma 後便) 個verb 要轉would

TOMING88 又亂答題目。

2012-01-11 05:31:14 補充：
004 is a good answer.

You had to diet yourself to become not so fat.
Hence restrict yourself to a diet.