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Grammatical Features of EST : Mood

一、Subjunctive Mood

In scientific and technical English,the subjunctive mood is widely used to describe hypothetical conditions. Hypothesis has two types-true hypothesis and false hypo-thesis. True hypothesis express the hope, wish and supposition of the writer. It indicates that the writer thinks that the condition is possible to be fulfilled. False hypothesis indicates that the writer thinks that the condition is unlikely to be fulfilled or it did not happen at all.

1. True Hypothesis

a. Possibility

In this case the hypothetical condition may be fulfilled. This type of hypothesis is used in scientific methods of research. It is used to reach conclusion. Example:

“A farmer has to cross a river one day with a dog, a sheep and some hay. The only boat he has is so small that he can only get one animal or the hay into it besides himself. The farmer knows that the dog is liable to attack sheep and that the sheep cannot resist eating hay. How does the farmer manage to get himself,the animals and the hay across the river safely?

The farmer now has several alternatives. Two of them are expressed in the following hypotheses :

First hypothesis; the farmer takes the sheep, leaving the dog and the hay together. What will be the consequence?

If the farmer takes the sheep, leaving the dog and the hay together, it will be a safe way.

Second hypothesis: the farmer takes the dog,leaving the sheep and the hay. What consequence can you deduce?

If the farmer takes the dog, leaving the sheep and hay, the hay will be eaten up by the sheep.

The farmer has to cross the river four times. He must take the sheep first. Then he takes the dog,returns with the sheep leaves (he sheep and takes the hay. Finally he takes the sheep and crosses the river again. That will be the safest way.

As can be seen from the above example, hypotheses are provisional explanations which are based on observation and can be tested by further observation. We can show this process as follows ;problem→observations→hypothesis (provisional explanation of relations between facts) →deductions from the hypothesis→evidence for or against the hypothesis→conclusion.

b. General Truths

Examples; If water becomes very cold, it freezes. If ice is placed in the sun,it melts.

These statements without reference to any particular time are true all the time. We use here the Simple Present tense in both clauses of the above examples.

2. False Hypothesis

a. Unlikeness

In this type of hypothesis, although the condition might happen, it probably will not. It is also used when referring to something that we regard as untrue. It is an unreal supposition.

Examples :

If a researcher performed an experiment (hypothesis), it might (could) be a failure (possibility only).

If all the ice in the world melted (possibility only), the level of the sea would rise about 250 ft (inevitable result).

If I were you, I should not do that. (untrue)

b. Non-occurance

In this kind of condition, we are referring to something in the past that did not happen. The supposition is impossible. Example :

If we had known the basic principles (we didn't know), we would have controlled the process even better.

二、Imperative Mood 

1. Instructions

The imperative mood is often used to give impersonal instructions to operators, employees , etc. It is often used for experimental or handling instructions, and in hypotheses or calculations.

Instructions may be given in two ways——

a. Using“should”

Examples: These machines should he handled with great care. Safety precautions should be observed at all times. The results of the experiment should be plotted on a graph.

N, B. This is sometimes used for politeness when“must be”is really meant.

b. Using the imperative form of the verb

Examples: Allow the water to cool for ten minutes and then take the temperature. Hold the convex lens is front of the white paper. Calculate the amount of expansion which will take place. Consider a steam chamber of high pressure and imagine its pressure to be x 1b/in². Let x equal the number of revolutions per minute. Suppose the water is drawn directly from a river. Assume that there is no Loss of heat from the boiler.

2. Advice or Recommendations

Instructions can be changed into advice or recommendations in order to explain how something should be done, or to explain the best or safest way of doing something.

a. Instructions

Examples: Grip handles as far as possible from blades. Hold cutting blades at right angles to workpiece. Align blades with cutting line. Do not completely close shears in one cut.

b. Advice or recommendations

Examples: You should grip the handles as far as possible from the blades. You should hold the cutting blades at right angles to the workpiece. You should align the blades with the cutting line. You should not completely close the shears in any one cut. Or: The handles should be gripped as far as possible from the blades. The cutting blades should be held at right angles to the workpiece. The blades should be aligned with the cutting line. The shears should not be completely closed in any one cut.

3. Rules

Recommendations can be made stronger by using "must" instead of "should". Strong recommendations often become rules, and therefore must be obeyed. Examples.

Rules for Brazing

a. The surface must be cleaned.

b. Flux must be applied.

c. The temperature of the work must be high enough to melt the spelter.

d. The two surfaces must be kept at the same temperature.

e. The work must cool evenly to prevent cracking.





1. 真实假设 

a. 可能性


例如:一天,有位农夫要带一条狗、一只羊和一些牧草过河。农夫唯有的一条船太小,除了装下他本人之外. 只能再装下其中一个动物或牧草。农夫知道,狗是要咬羊的,羊又是忍不住要吃草的。那么,这位农夫怎样才能设法使他本人还有那两个动物和牧草安全过河呢?








b. 一般真理



2. 非真实假设 

a. 不太可能发生的事





b. 没有发生的事




1. 指示或命令



a. 用 “should”(应)


注意:为了礼貌起见。在实际意义上该用must be (必须)的场合,有时也用这种形式。

b. 用动词的祈使式

例如:让水冷却十分钟后再测水温。在白纸前方架一个凸透镜。计算将发生的膨胀量。考虑一个髙压蒸汽室, 并想象其压力为x磅/英寸²。令x等子每分转数,假定直接从河里抽取水。餒定锅炉没有任何热损失。

2. 劝告或建议


a. 指示或命令


b. 劝告或建议



3. 规则



a. 焊接面必须淸洗干净;

b. 必须使用焊剂;

c. 工件的温度必须高到能熔化焊锡;

d. 两焊面必须保持相同的温度;

e. 工件必须均匀冷却以防破裂。


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