いけ+ば

So in order to put my novice vocabulary to use, as well as to keep it polished, I decided to put more time into actual reading.
I decided to make a gentle start, and as such picked up a couple of fairy tales that came with translations.

1) むかし、 むかし、 ある ところ に
2) おじいさん と おばあさん が いました。

3) じいさん が 山（やま） へ 木（き） を きり に いけば、
When the old man went to the mountain to cut trees,

4) おばあさん は 川（かわ） へ せんたく に でかけます。
the old woman went out to the river to wash clothes.


（This is a quote from Momotarou）


The thing I'm not getting, is how the ば at the end of the first sentence can mean "when".
I am aware of e+ば meaning "if", as well as the meaning of "ば いい", etc. However, I'm failing at finding any info on this case in particular.



Also, why do they use は instead of が when they're talking about the old lady? Is it like this?:

"If/when (still vague on this) the old man went to the mountains for cutting wood, (as for) the old lady went to the river to do the laundry." ?

So because the topic shifted from the old man, to the old lady?
I'm confused because in the sentence before this, "the old lady AND the old man" were already declared as topic.

If that's the reason, doesn't that mean that SUBJECT marker が in sentence 3 shifted the TOPIC from the couple, to simply the old man?

So が can actually change the topic, despite not being a topic marker?

The -ba conditional can express "when" when it's used for a habitual action, especially a past habitual action.
e.g.
会えばけんかしてたね。長く居すぎたのかな。
(The nuance is more close to 会う度にけんかばかりしてたね "every time we met, we always had a quarrel" here, though.)

は performs as a contrastive marker in the last line.

Thanks! =D I couldn't find any info on that no matter where I looked!