We didn’t feel that our feet were aching until we wengot home and sat down.

No wonder some of my friends asked , ''WhyHow can Iyou walk for such a long time?''

This is more how we would say it in the US.

Unfortunately, the sizes of them are small clothes were too small but I still found some books in which I’m interested.

I'm not sure what you meant so this is my best guess.

or,

- Unfortunately, the sizes of the clothes were too small but I still found some books which I found interesting.
- Unfortunately, the sizes of the clothes were too small but I still found some books which interested me.

But I still found some books which I’m interested in.

This is a sentence fragment.

Most of them probably can be found at the library of my university where I have studiedbeen studying.

You're still studying at university, right?

We seldom go to that department store, even though it is near my house.

We didn’t feelrealise our feet were aching until we went home and sat down.

Or: "we didn't feel our feet aching until we went home and sat down."

No wonder some of my friends asked ''Why: ''How can I walk for a long time?''

They want to know "how you are able to do something", not "why do you do something".

Unfortunately, the sizes of them aremarket and fair were only small.

Alternatively: "Unfortunately they were only small", but the subject isn't clear (you could also be talking about your lunch!).

But I still found some books which I’mthat I was interested in.

Although you (hopefully!) are still interested in them, you are talking about something that happened in the past (finding the books).

Most of them can probably can be found at the library of my university where I have studiedy.

You are still studying, so this should be present tense.

HenceSo, I will borrow them when the new semester starts.

"Hence" is too strong here. You could still buy the books, even though they are available in the library (maybe you want to own your own copy!).