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。
（1）在前言后半部分，用转折结束文献回顾，指出尚未研究的问题或不足之处
e.g. 转折副词+指出过去研究的不足+研究主题
However,few studies have been done on/ few studies have been reported on/ few researchers have studied/ little research has been devoted to/ little attention has been paid to + the supercritical fluid extraction of oil. 需看主语注意单复数。
e.g.连接词+主题一的研究程度+主题二的研究程度
Although+ much research has been devoted to A+ little research has been done on B.
Although+ much work has been done on A+ little attention has paid to B.
While+ much literature is available on A+ few studies have investigated B.
(2)提出本文的研究目的或主题
e.g.研究导向型：一般过去时+研究主题
The purpose of the experiment reported was + to determine …
The aim of this study was+ to examine the …
The objective of this research was+ …
In this study, we determined a new method …
e.g.论文导向型：一般现在时+研究主题
The purpose of this paper is + to obtain …
The present paper reports/discusses + …
This paper proposes/presents/describes+ …
In this paper, we explore/provide/discuss+ …
(3)表述本文的研究价值
e.g.理论价值
Our results may help to clarify whether… is effective.
The results of this study may help to explain how… can be…
e.g.应用价值
The methods reported here could be beneficial to research attempting to increase…
The results obtained from this novel approach would provide better insight to …
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