ga vs. ha in this context

おじいさん が 山（やま） へ 木（き） を きり に いけば、
おばあさん は 川（かわ） へ せんたく に でかけます。

I read this children stories on the Internet and I came across this sentence which involves the use of が and は. My question is why the first part of uses が and the second part uses は。Is it because of いけ which is a potential form or いけば which is a conditional form? Moreover, why is the latter part uses は any explanation?

normally,
おじいさんは山(やま）へ芝刈（しばか）りに（行きました。）、
おばあさんは川（かわ）へ洗濯（せんたく）に行（い）きました。
Grandfather turned the lawn to the mountain,
Grandmother went to the river to do the laundry
I remember this way.
" went（行きました）" is a repetitive word, so I omitted it the first time.
After "は" is a noun or adjective followed by.
It seems that the verb will follow after "が".
There are cases where this is reversed.
It seems to be used to identify something then.
https://www.alc.co.jp/jpn/article/faq/03/19.html

You need to add "if, as, when, or while" as the condition by "ikeba" ending in the first sentence, followed by the next sentence.
If, As, When, While I do this, you do that.