It's too hard to study all day.

Currently, I'm studying all day, but I can't tell why I'm studying so hard like this.

[It's more natural to say "I'm currently studying all day, but I can't tell why I'm studying so hard like this"]

Studying is hard because the weather is so hot in these days.

I repeat am turning the air conditioner on and off the air conditioner repeatedly.

It's too hot when I turn it off and it gets cold when I turn it on..

And I'm curious that what about how much my electric bill will cost this month.

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You have written it well! Good job :)