English sentence structure

希禮：
to keep an eye on 或 keeping an eye on 是小心看護一人或物免被損壞的意思, 並不是觀看的意思。你要多留意這些基本的英文用語常識, 普通的學生字典如Oxford Advanced Learner's Dictionary 裏在eye字一項中便解釋得不錯, keep an eye on somebody/something means to take care of somebody/ something and make sure they are not harmed, damaged, etc., like " We've to ask the neighbours to keep an eye on the house for us while we are away.", 所以我(估)認為你是想說的是 " The only thing you have to do is to watch the film and the subtitles (thereof) at the same time."。
一齣電影有多句旁白, 所以要用眾數subtitles。在film 和subtitles之前要加個article冠詞the,因句子含義是觀看着該片及片中的旁白, 所以film 和subtitles前要加 s句子文法才對。

文法唔可以話係錯很多，但係句法上用錯了is keeping (gerund)這個字和keep an eye這詞的意思用錯了。再者film和subtitle都要加the.

先講句法的應該是：
The only thing you have to do is to keep an eye on ....

用keeping(gerund)的意思是表達一項真理(不變的現象/情況)，明顯這句是命令句，指示一個做一樣事，所以have to do的是is一個行動而不是一項情況或真理。所以要用to infinite作為子句。這是用意決定文法的用途。

第二，keep an eye的意思是「睇水」或「留心/看護」-例如你媽叫你在家看住個bb或弟妹就叫做keep an eye on the baby/your brother/your sister，而不是「觀看」或「注意」。所以叫人看護或「睇水」住電影和字幕並不是這句的意思。
正確的用詞應該是to watch觀看或to pay attention留意/注意。

第三，這句中film和subtitle(s)都是講者心目中指定的東西，加上這些字本身必須要有article(a/an, the)才合文法的。

所以表達好一點的寫法是：
The only thing you have to do is to pay attention on the film and the subtitles.

Corrected and better version:
"The only thing you have to do is keep an eye on the film and subtitles.
Explanation:
Using a gerund, keeping, after the phrase "to do is" is very old-fashioned English; I am not sure whether it is acceptable grammatically nowadays.
As you can see contemporary English usage uses "infinitive without to" after the phrase ..."to do is" which corresponds with "to do is"