前言通常分四个步骤：1.背景资料，2.文献回顾，3.指出问题，4.介绍自己的研究活动。下面分享下，前言写作中常用句型与时态，以供参考。

一、背景资料

（1）背景资料中指出作者专业领域里最近发生的某种趋势，或最近发生的某个事件，则使用现在完成时态。

e.g. The lithium-mediated system of electrochemical nitrogen reduction has emerged as a promising alternative to the highly carbon-intensive steam methane reforming and Haber-Bosch process for manufacturing ammonia.

（2）背景资料中，叙述有关某些现象或某个研究领域的普遍事实，用一般现在时。

e.g. One of the byproducts is natural gas, which is comprised overwhelmingly (79%-90%) of methane.

二、文献回顾——引用文献举例

（1）从句中的资料为普遍有效事实/技术或标准，用一般现在时。

作者+动词（一般过去时）+that+研究结果（一般现在时）

e.g. Marks et al. + showed/found/reported/noted +that +water boils at 100 ^oC.

（2）从句中的资料仅在特定情况下才有效。

作者+动词（一般过去时）+that+研究结果（一般过去时）

e.g. Smith et al. found/observed/indicated+ that the predicted concentration were agreement with the experimental data.

e.g. Further complication comes from the observation by Tatara et al., who showed that the discharge capacity of Na-O₂ cells with sodium bis(trifluoromethanesulfonyl)imide (NaTFSI)/DME increased when going from room temperature to 10 °C, which is (使用现在时分享你个人对有问题的研究的观点) in contrast to the trend of the discharge capacity of Li-O₂ cells over the same temperature range.

(3)从句中只表示一种建议或假设

作者+推测动词(一般过去时)+that+资料（may+动词原形）

e.g. Lin et al. suggested/hypothesized/proposed/argued +that +the NH₃ gas may spontaneously react with the carbon species.

(4)对引述资料的评语：针对过去的行为和事件，用一般过去时，不受时间影响的事实，用一般现在时。

e.g. Rahil et al. proposed a model based on …With this model, the variations of porosity could be described.

e.g. Azis took into account the effect of ….However, his analysis was restricted to the …

e.g. John et al. presented a method for solving problems…However, this method is not applicable to the general case.