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.
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