不是就用英语怎么说

“不是就”在英语中常用短语是 "otherwise" 或 "if not"。
"Otherwise" 意思是 "in a different way; on the other hand"，如果不是这样的话。 例如：
I'll finish the work by tonight, otherwise I'll have to stay late. (如果不是这样的话，我今晚会完成工作。)
He'll be late, otherwise he would have been here by now. (如果不是这样的话，他现在已经到了。)
"If not" 意思是 "if it is not the case that" 或 "if something does not happen"，如果不是这样的话。 例如：
If not for your help, we would not have been able to finish this project on time. (如果不是你的帮助，我们不可能按时完成这个项目。)
I'll take the bus, if not I'll walk. (如果不是这样的话，我会步行。)