The Talmud states clearly, “Borrowing an object without permission is tantamount to stealing it.” Therefore, generally speaking it is forbidden to borrow such items. If they were taken, they must be returned immediately as is the law for a stolen object.
The above is the general rule. However, when one is absolutely certain that the person would not mind, the commentaries to the Talmud say that the above statement doesn’t apply. The above ruling is referring to a standard case where one does not know if the person minds or not. In a situation where one knows the other person doesn’t mind, then it depends upon what type of object one is borrowing.
If it’s the type of object that will be used up partially, like toiletries, food, etc., it’s better not to use those items unless it’s a case of great necessity. If the items are the types that remain intact after their use, like tools and the like, and one is certain the owner doesn’t mind, it is permitted. This is usually established by the fact that the friend has been granted permission in the past by the owner. The authorities have ruled that the use of a pen falls into the latter category of items remaining intact, although a small amount of the ink is used up by its use.
If it is common knowledge that the owner doesn’t mind if anyone uses his object it can be used without permission.
Similarly, the Talmud rules that the average person doesn’t mind if another uses his mitzvah object without permission, such as to don his tefillin or tallis (provided the owner doesn’t need them at that time and they will be returned undamaged). This is based on the statement that “a person is happy for another to fulfill a mitzvah with his objects”.
In situations where there is an established custom to allow others to share of their food, such as roommates in a dorm, no explicit permission need be sought.
Rabbi Yerachmiel Fried