nssa guide is helpful about that. Look here:(i have added/changed some things i feel are important as well)
General editors must be fluent in English and be very familiar with the grammar and punctuation conventions used in English.
To respect the contribution of the subbers, please avoid making any unnecessary changes. For example:
Do not edit the subtitle only to add a break unless a break is necessary.
If the original sentence is correct in terms of grammar,syntax, punctuation, and make sence in English then please do not change the sentence.
BE AWARE OF THE GUIDELINES FOR SUBBING SO THAT YOU CAN MAKE APPROPRIATE CHANGES OR ADDITIONS
Please be aware of where italics, music symbols, round brackets and square brackets should be used to be able to add these in when editing.
These can be found in Chapter III.
Please do not try to rearrange a grammatically correct sentence as it may change the meaning of the sentence. This is especially so for Chinese. A small change in the wording of a sentence could change the whole meaning of a subtitle. Therefore, please assume that the original subtitle is correct and make changes where needed only.
GRAMMAR AND PUNCTUATION EDITING
General editors are assumed to have knowledge of correct English grammar and punctuation. This includes the correct phrasing of a sentence, correct tense and correct positioning of full stops/periods, commas, questions marks and speech marks.
These will need to be included where needed and also follow the guidelines in Chapter III.
http://nssacademy.weebly.com/subtitling-guide.html#guidelines