One day I was walking with a colleague who was carrying a cup of hot coffee. I saw that it was about to spill. What should I have said? Watch out! You’re about to spill your coffee! Be careful or you’re going to spill your coffee!

Walking with my colleague one day, I noticed that coffee was about to spill out of his cup. So I said to him: "Be careful! Your coffee is going to spill." "Spill" is a perfect word. "Drip" is not a good word. You would use the word "drip" if the cup had a hole through which the coffee was falling one drop at a time. You can say any of these: "Your coffee will spill" "Your coffee might spill" "Your coffee is about to spill" "Your coffee is almost spilling"

"Or are there any other better expressions?" or "Any other better expressions?" sound more natural. There are a lot of things you could say in this situation. For your first example, the following sounds a bit more natural: "Be careful. Your coffee is about to spill." For the second example, "drip" is unnatural in this situation. If "drip" were to be used, you might say, "Watch out! Your coffee is dripping!" ("down the side of the cup" is implied) Other things that sound natural in this situation: - "Be careful! Your coffee!" (while you gesture to the cup about to spill) This phrase just brings the coffee to your listener's attention. - "Careful, the coffee's going to spill!"