Symbol for 'When ...' I have never found one listed anywhere: Is there a symbol for a 'when' condition (example below).
$A=B$ when $A$ is even
 A: This is the same as "If $A$ is even, then $A=B$", or symbolically "$A$ is even $\Rightarrow A=B$". So, if you really wanted to keep the order as you have it, you could write "$A=B \Leftarrow A$ is even", but you run the risk of others not knowing quite what you mean.
A: It's an implication: ($A$ is even) implies ($A=B$), or
$$(A \mathrm{\ is\ even})\implies(A=B)$$
A: If you want to keep them in the original order, you can write "$A = B$ if $A$ is even."  This is semantically the same as the other answers.  Of course, "if" is not a symbol per se, but it might as well be one, and it's a little clearer and more directly logical than "when".
A: As said, $A=B\Leftarrow A\text{ is even}$, or $A=B\text{ if }A\text{ is even}$ works, but we can do it slightly differently too; "because" also means the same thing as "if" or "when":
$$
A=B\quad\because\,A\text{ is even}
$$
Personally, I see "$A=B\text{ if }A\text{ is even}$" the most (and yes, I actually see this quite a bit; my professor likes to state theorems with "if"), and I like to explain why I skipped a bunch of steps on my homework by saying "because" (although I usually don't use $\because$ because it looks a lot like $\therefore$, but is much less used)
