I'm curious about whether to use comma before "and". Some people told me that using comma to connect two different sentences and two different subjects.
Please provide some examples to explain the usage of this. Thanks a lot!
Generally we don't use commas before AND. Since Commas are used in pairs to enclose phrases that interrupt a clause or that are intended to function parenthetically, a writer may choose to place a comma before "and" (or any of the seven coordinating conjunctions) when the conjunction launches such a phrase:
The issue is whether and is joining a compound verb or a compound sentence. There should not be a comma before and when it joins a compound verb: Alice picked up one of the books and read it.
Usually a comma should be used before and when it joins a compound sentence:
Alice picked up one of the books, and then she walked out the door with it.
You may use the clue that a noun or pronoun follows and, signalling the start of a second clause (which is what makes the sentence a compound one).
The comma may be omitted, however, if the thoughts of the two clauses are closely related:
Alice picked up one of the books and she dropped it immediately.
If the word before the 'and' is a noun, and the next word is not a noun (or a noun phrase), use a comma. Otherwise you are combining things that don't belong together.
'I want some bread and wine'
'I want some bread, and could you bring me some wine too?'
'I went to get a paper and a cigar'.
'I went to get a paper, and to take a walk'. (Not 'a paper and take'. What is a 'take'??!!!)