Tip: See my list of the 
Most Common Mistakes in English. It will teach you how to avoid mistakes with commas, prepositions, irregular verbs, and much more.
Most languages don’t require a comma before “and” when it joins two independent clauses, but writing a comma before “but”, “so”, and other conjunctions is quite common. In English, however, there is no difference between “and” and other conjunctions in terms of commas—if they join two independent clauses, you should almost always write a comma:
correct I had to go to the airport, so/but/and I couldn’t attend the party.
wrong I had to go to the airport so/but/and I couldn’t attend the party.
correct She’s already seen the film, and she doesn’t want to go.
wrong She’s already seen the film and she doesn’t want to go.
I wrote “almost always” because it is usually considered acceptable to omit the comma when both clauses are very short, e.g.
correct I played the guitar and she sang. (acceptable)
correct I played the guitar, and she sang.
The solution seems to be simple: Just write the comma every time, and you cannot be wrong. But, there is a catch. When “and”, “but”, and other conjunctions separate just two words, not two clauses, we don’t use a comma, as in
correct I like apples and oranges.
unnatural I like apples, and oranges.
The problem is that verbs are also just words. If a conjunction connects two verbs in a single clause (that is, if there is only one subject), there is no comma before it:
correct He cooks and eats.
wrong He cooks, and eats.
correct He can ride a bike but can’t swim.
wrong He can ride a bike, but can’t swim.
This can get confusing when the sentence you analyse is long. As long as there is no subject in what seems to be the other clause, you shouldn’t use a comma:
correct She cooks for the whole family and eats a lot of vegetables.
wrong She cooks for the whole family, and eats a lot of vegetables.
Theoretically, you could avoid this situation by repeating the subject, but this can hardly be recommended; you will certainly agree that the following sentence sounds a bit clumsy:
She cooks for the whole family, and she eats a lot of vegetables.
This sentence may be grammatically correct, but the variant without the second pronoun sounds much more natural.
