I studied the wholeall day at the library. I studied all day at the library.

"I studied the whole day" is understandable, but somewhat vague. Did you study "The day" or did you study "all day".

I studied the whole dayThroughout today, I studied at the library. Throughout today, I studied at the library.

I studied the wholeall day at the library. I studied all day at the library.

WThen whole time I was there, I always felt at peace. The whole time I was there, I felt at peace.

“When” and “always” make your original sound like a description of something that happened a long time ago.

When I was there, I always felt at peace. When I was there, I always felt at peace.

I felt at peace the whole time that I was there.

Whilen I was there, I always felt at peace. While I was there, I felt at peace.

Or you could say "When I am there, I feel at peace", "I always feel at peace when I am there".

When I was there, I always felt at peace. When I was there, I felt at peace.

Whilen I was there, I always felt afelt peace.ful While I was there, I felt peaceful

I’m going to finishwrite my last major test soon! I’m going to write my last major test soon!

Good luck.