Use a comma between independent clauses (each clause has a subject and a verb) in a compound sentence.
These are usually separated by a conjunction (and, but, or) and a comma.
We had been wanting to buy it for years, but the owner always seemed reluctant to sell it.I had to go to the store to buy bread, and she decided that she wanted to go.
Note that there are many ways that a sentence can be punctuated — it depends on how strongly you want to set off two or more items in a sentence. In these sentences you could also make two separate sentences or use a semicolon (;).
If the clauses of the sentence are short, or if the ideas in the two clauses are closely connected, a comma is not necessary.
I saw them and they saw me.
subject + 2 verbs: no comma is necessary (usually)
Sentences with one subject and two verbs usually don't need a comma.
I went to the store and bought a can of cat food. ("I" is the subject of both verbs.)
If the predicate is lengthy and complex or if one part of the sentence is being emphasized, the sentence can take a comma.
His idea was the worst, and has been a source of confusion and frustration to all involved.
(Compound predicate, but a comma is used for emphasis and clarity.)
This point is really a matter of what you want to express. If you feel a clause is lengthy, or if you want to clearly set it apart from another part of the sentence for emphasis, use a comma. If you feel both parts of the sentence are more or less equal, you can omit the comma.
Exercises
1) Click the "Original" button and make any necessary changes to the sentence. (Type or remove the punctuation marks in the box.)
[Original]のボタンをクリックすると下のスペースに文章がでてきます。 まちがっていると思う箇所を訂正しましょう。（コンマなどの句読点マークを入れたり、削ったりして下さい。）
2) Click "Check!" to check your answer.
[Check]のボタンをクリックし答をチェックしてみましょう。
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