一般过去时及其被动语态
An analysis of the cure kinetics of several different formulations composed of bifunctional epoxy resins and aromatic diamines was performed. …… All kinetic parameters of the curing reaction were calculated and reported. Dynamic and isothermal DSC yielded different results. An explanation was offered in terms of different curing mechanism which prevail under different curing conditions. A mechanism scheme was proposed to account for various possible reactions during cure.

现在完成时及其被动语态
The partial molar enthalpies of mixing of NaHSO4 and KHSO4 have been measured at 528K by dropping samples of pure compounds into molten mixtures of NaHSO4 and KHSO4 in Calvet calorimeter. From these values the molar enthalpy of mixing has been deduced.
The phase diagram of this system has been confirmed by conductometric and thermal analysis methods. By an optimization method the excess entropy of the liquid mixtures was also calculated.

应理解为上述时态是撰写摘要时常用的几种时态，有时很难区分它们在含义上的严格差异。

这只是从语法功能的角度将其概念化，实际写作英文摘要时，这几种时态并非完全不可互易。

【转引自】http://www.pet2008.cn/thread-16595-1-2.html