Today’s idiom is based on sports. It is to “be a whole new ball game”. We use it when we want to talk about when we have a bad situation and then change it in some way. Then, this change results in an improved situation. For example:
I used to hate going to work, but my old boss left the company a few weeks ago. After that, it was a whole new ball game.
My husband and I had a lot of problems in our marriage, so we went to a marriage counselor. Since then, it’s been a whole new ball game. We are much happier with each other now.
My class has the lowest grades in the whole school, but it’s because we had a terrible teacher. Now, we have a really good teacher, and it’s a whole new ball game.
Jeff wasn’t a very good athlete, but he’s been exercising a lot lately so it’s a whole new ball game.
If the situation has just changed and the improvement hasn’t yet happened, but we think it will happen in the future, we say “It’s a whole new ball game.” with the present tense. If the improvement has already happened, we say, “It was a whole new ball game.” with the past tense.
I think this idiom is based on the idea in sports, especially baseball, when a team is losing a game. Then, they change the players and suddenly they start to win. I hope this is clear to everyone.
English Help Online's Blog 
kengo Said:
on March 29, 2011 at 3:59 pm
Hi Mike.
I want to ask you about the tense.
Do you use the expression with a present perfect tense like the second
example when you want to talk about something still getting better?
Or, do you use it when the result still continues?
Thank you.
englishhelponline Said:
on March 31, 2011 at 5:57 am
Hi there.
The reason why I used “it’s been a whole new ball game” in that example is because of the word “since”. We usually use “since” and the present perfect together. In this example, it means that the situation has already changed. It doesn’t mean that it is in the process of changing.
Mike