今天奶奶生日用英语可以说成 "Today is my grandmother's birthday."

"Today" 表示今天,"is" 是助动词,表示现在的动作或状态,"my grandmother's" 表示我的奶奶,"birthday" 表示生日。

下面是一些例句:

"I'm going to buy a birthday present for my grandma today." (我今天要给我的奶奶买个生日礼物。)
"Happy birthday to my dear grandma!" (祝我亲爱的奶奶生日快乐!)
"I'm going to call my grandma and sing 'Happy Birthday' to her." (我要打电话给奶奶,并为她唱生日快乐歌。)

希望这些例句能帮助你。