Tom finished a lap ahead of his team-mate.

句中的 aheadof his team-mate 算是介詞短語嗎？

aheadof his team-mate 當然是介詞短語。

aheadof 是“復合介詞”，由其構成的短語，同樣是介詞短語。

“復合介詞”是一種固定搭配，是一種意義的合成，兩者缺一不可。

比如：

ahead of 在...前面

up to 接近；達到

close to 挨近

out of 從...中出來

away from 離得遠遠的...

north of 在...以北

instead of 而不是；取代

regardless of 不管；盡管

according to 依據

because of 由于

owing to 由于

due to 由于