在沙漠上躺着可以用英文表达为 "lying on the desert" 或 "laying on the desert." 两者的区别是 "lying" 是不及物动词，表示正在躺着的状态，而 "laying" 是及物动词，表示将某物放置在某处的动作。

例句：

The sun was beating down on him as he lay on the desert, completely exhausted. (他躺在沙漠上，完全筋疲力尽，太阳炙烤着他。)
She laid her backpack on the desert floor and sat down next to it. (她把背包放在沙漠地面上，坐在旁边。)

"Lie" 和 "lay" 在英语中都有不同的含义和用法，它们的过去式分别是 "lay" 和 "lain." 常见的用法包括：

"Lie" 表示"躺"，作不及物动词时，过去式是 "lay"，过去分词是 "lain." 例如：

I lay down on the couch and closed my eyes. (我躺在沙发上闭上眼睛。)

He had been lying in bed all day, feepng sick. (他整天躺在床上，感觉不舒服。)

"Lay" 表示"放置"，作及物动词时，过去式是 "laid"，过去分词是 "laid." 例如：

She laid the book on the table. (她把书放在桌子上。)

I laid the baby in his crib and kissed him goodnight. (我把婴儿放在婴儿床里，吻了他一个晚安。)

希望以上内容能帮助到您。