again, over again, once more, once again 都表示“再次，又一次”的意思，用来指重复一次同样的动作。如：

I gave her medicine again at 5:00. 五点钟我又让她吃了一次药。

Let’s try once again/over again. 我们再试一次。

Do it once more. 再做一次。

now and again 意思是“不时地，间或”，是个“频度副词”。如：

Now and again I saw John walking along the river. 我时常见约翰沿河边散步。

over and over again, again and again, time and time again 则表示“一再地，多次地，反复地”的意思，用来强调重复做某事。如：

I've warned you over and over again not to do that. 我一再地警告你不要去做。

He makes the same mistake again and again. 他一再犯同样的错误。

He had asked himself that question time and time again. 他反复问自己那个问题。