No "the" in "on large screens" because we are thinking about "large screens" in general, not a specific set of them e.g. the large screens that had been distributed earlier. "on screen" is an adverbial phrase that modifies "make changes". It gives detail about where and how to make changes. It is similar to "in writing", or "by email". Nouns are singular and without articles in such phrases.

Tests often focus on knowledge of rules, rather than knowledge of language. This is a shame: the rules of English don't tell the whole story. One day, when your exams are over, you will be free to learn the whole story. I think you have done well to find the 'zero article' rule as an explanation for the 'correct' answer; Michael agrees. In truth, you are right to be puzzled: there are contexts where both variants, with and without 'the', are acceptable, sometimes one is better, sometimes the other: only the context tells us which. Exams rarely give context, you have to use your knowledge of examiner psychology to figure it out. If the exam always has a Zero Article question, you have to learn how to recognise it, and give the examiner what he wants.

If something you are gonna talk about is already mentioned,you can use "the". If it is not the case,you need to use "a" or "an"

The flaw in my English usage is where I can be better, only if when I know 1. Thanks for your reply again, and your suggestion. I thought I'm trying to follow your instruction to use English correctly, and try to solve the problem I faced (e.g.check the rules ), before raising my question. 2. My original thought in grammar is like what you reply. But the reason I tend to use "the" in this article is by logical thinking, by inference, if there is no other grammar rule here. (1)According to the context, a person went to a concert at a Park, and saw the crowd all on site. "Others were watching the performance on large screens." Here "Others" should still mean people who were present at the concert. So It's more reasonable for me to think "the large screens" is meant that had been placed there, or at least it should be limited screens on site. The point is If I'm the person at the concert, the screen I see must be on site. Otherwise, not in this case, If "others" mean people are not present, "big screen" could mean "TV or youtube screen..." So the above is the reason I think it should use "the"=> "on the large screens" If there is anything wrong, please correct me. It will be appreciated.

As Jimmy suggested, the difference is quite subtle, and typically, as he suggested, you have to absorb how to use the language correctly, rather than learn rules. In this case, since which screens have not been explicitly defined, then they are general, and therefore you don't use 'the'. Conversely, 'on the large screens that had been placed around the square'. Here, they are explicitly defined...