A,B are two spaces apart（A,B相隔为2） (A□B) or |A－B|=2

* Two spaces separate A and B = A,B are two spaces apart

又如：A is two floors above B, 则A和B只有一层相隔

又如：A is two days before B, 则A与B之间只有一天之隔

区别：A,B之间有两个元素 (A□□B)

A is to the right of B: B－A

A is to the immediate right of B: AB

B is to the right of either A or C, but not both: (A-B-C) or A/C——B——A/C

区别：B is between A and C: A－B－C

区别：B is immediately between A and C: (ABC)

=B is (immediately) next to both A and C.

If A is next to B, it is not next to C: ~(BAC)

2.表选择

If A, then B（A入选则B入选） A→B 逆反命题 -B→~A

选A则不选B A→~B （但可同时不选）

=A,B不全选 A→~B 逆反命题 B→~A（区别A,B全不选）

同理：A灯亮则B灯灭 A→~B 逆反命题 B→~A （但A,B可同时灭）

另外：A is off if and only if either B or C is on: B/C→~A （B和C可同时亮时A也不亮）

逆反命题 A→ ~B/C （A亮时，B,C都不能亮,即neither A nor B is on=both off）

If A, then B, and if B, A: A<——>B

=If A, then B, and if ~A, ~B

=A if and only if B

=B if and only if A

=Neither A nor B unless the other is selected.

A only if B: A→B 逆反命题 B→~A

A unless B: ~A——>B 逆反命题 ~B→A

区别：Not A unless B: A→B 逆反命题 B→~A

Only A works in the afternoons: only A=pm

区别：A only works in the afternoons: A=pm

If A occurs in a word, it occurs an odd number of time: J→J=odd#

Only As are Bs: B→A

=only if A, B

No As are Bs. A≠B （A,B无交集）