the thing is idiom
B2 informal问题是
used to introduce an explanation or an excuse
The thing is, my parents like me to be home by ten o'clock. 问题是，我父母想要我10点前回家。
The thing is, the shops close early on Sundays. 问题是，商店星期天很早就关门。
要紧的是
used to emphasize the importance of what you are saying
The thing is to keep a good distance from the car in front of you, when you're driving in snow. 要紧的是，在雪地驾车时，你要和前面的车辆保持很远的距离。