认真上课用英语可以说成 "pay attention in class" 或者 "be attentive in class"。这个短语来源于英语中 pay attention 的意思是“注意”，而 attentive 的意思是“专心的”。
例句：
"It's important to pay attention in class if you want to learn and understand the material." (重要的是认真上课如果你想学习和理解材料。)
"I always try to be attentive in class so that I can take good notes." (我总是试着认真上课,这样我就能写好笔记。)