The countdown for the rocket launch has already started.
Tips
mistakes
The second character decides which word this is. Counting down is 倒数, with 数 as the verb to count. In maths the same two characters are read 倒数, a noun meaning the reciprocal of a number.
usage
倒数第一 means 'last place': literally 'first counting from the bottom.' Chinese students dread hearing 倒数第一 on test results. It's a much more vivid (and slightly more humiliating) way of saying 'last' than just 最后.
