HONEYMOON 新婚旅行

A honeymoon is a vacation trip taken by a man and woman who have just gotten married.

To BE "on a honeymoon" is to be on such a trip. To GO "on a honeymoon" is to take such a trip.
honeymoon は、結婚したての男女が行く旅行、つまり、新婚旅行のことです。

BE on a honeymoon は、新婚旅行中、GO on a honeymoon は、新婚旅行に行く、という意味です。

I had such a wonderful time on my honeymoon! It was the best trip of my life.
a: Is Tatsuya back yet?
b: No, he's still on his honeymoon.
Where are you going on your honeymoon?
Bob and Sarah went to Maui on their honeymoon.
We're not going on a honeymoon. We've decided to use the money to buy a house instead.

Come back again next week for the third chapter of our "Boy Meets Girl" special. And remember, you can only see it here, at Berlitz WordMaster!