What is the ruling on damaging borrowed items of little value? Am I liable for a guarantee if the owner does not demand it from me? How should I deal with old borrowed items whose fate I cannot recall and whose owners I cannot contact? What should I do about a pen I took from a teacher, which belonged to a student, and which I could not return to its owner or to the teacher?

If borrowed items are damaged due to negligence or transgression, then compensation is required. If they are damaged without negligence, scholars have differed regarding compensation, but the preponderant opinion is that it is obligatory. You must return an equivalent of the items or their value on the day they were damaged if an equivalent is not possible. It is permissible to seek forgiveness from their owners. If you cannot reach them, then give charity on their behalf with an equivalent of the items or their value. If you doubt whether you have returned what you borrowed, the fundamental principle is that it remains your responsibility.