虚拟语气（5）
3、出现在状语从句中的虚拟语气形式 (1) 出现在由“as if [= as though]（好像）”引导的从句中(少数人不用虚拟语气的；如果某事很可能发生则不用虚拟语气) 其形式是： 如果涉及现在：“be”用“were”（也有人用“was”的）；其它动词用过去时 如果涉及过去：谓语动词用过去完成时 Each of the two wires(导线) carrying (携带) currents has a magnetic field(磁场), as if it were a magnet(磁铁).
We have for simplicity(简单) been speaking of(把…说成) the current in a conductor(导体) as if all of the free electrons(自由电子) moved with the same constant velocity(恒速). (注意：“speak of … as”与“as if”共用了“as”) In this case, the leaves(叶片) of the electrometer(静电计) will diverge(发散开) just as if they had been charged (带电) from a body(物体) electrified (带电) by contact(接触).
(2) 出现在由“whether … or …”引导的让步状语从句中 这主要出现在从句中使用动词“be”的情况（不过有人在该从句中是不用虚拟语气的）。 The same basic concepts(基本概念) apply to(适用于) all types of feedback control systems(反馈控制系统), whether the controlled variable(受控变量) be temperature, speed, pressure(压力), flow(流量), position (位置), force, torque(力矩), or any other physical quantity(物理量).
重要的是，最常见的情况是省略“whether”时：从句一定要采用be it [they] … or …，其中的“it”或“they”指的是主句的主语。 Everything around us, be it air, water, or wood, is matter(物质). All such problems, be they easy or difficult, can be solved by a computer.
(3) 出现在由“lest”，“in case”，和“for fear that”等表示“以免”的从句中以及由“in order that”，“so that”表示的目的状语从句中。 其谓语形式常用“（should）+ 动词原形”。如： Batteries(电池) should be kept(放置) in dry place(干燥处) lest electricity (电) leak away(漏掉). In order that this expression(表达式) be a definite form(定界形式), the necessary and sufficient condition(充分必要条件) is as follows(如下). This equation(方程) holds(成立), no matter what [= whatever] the independent variable(自变量) be.
4、在主句或某些从句中的虚拟语气形式 这种虚拟语气主要表示主观推测或语气委婉。其谓语形式为： “should/would/could/might +动词原形” It would not be difficult to prevent (防止) road accidents (车祸) with the help of electronic apparatus(电子设备). One might think(认为) that friction (摩擦) is always undesirable(不希望的). Since(因为) the limit(极限) is not greater than unity(一), the series(级数)diverges(发散) although at first sight(初看起来) it might appear(似乎)to converge(收敛).