“先在”的英语可以翻译为：preexistence ...

我先在画布上涂了一层蓝色水彩。First I applied a blue colour wash to the canvas.你最好先在地上洒一层水,然后再打扫.You'd better damp down the ground and then sweep it.先在外面等一会儿.Wait outside the house for a while....