Since past tense of a durative verb -express a negative meaning It is three years since he was in the army. It is a long time since I studied english (4)when, while, as 当无线电波沿地球表面传播时,会损失掉一部分能量。 As radio waves travel along the surface of the earth, part of its energy will be lost.Since + past tense of a durative verb ----express a negative meaning It is three years since he was in the army. It is a long time since I studied English. (4) when, while, as 当无线电波沿地球表面传播时，会损失掉一部分能量。 As radio waves travel along the surface of the earth, part of its energy will be lost