Using someone’s item without permission:
It is forbidden for one to use an item that belongs to one’s friend without his permission. This applies even if one only plans to use it on a mere occasion and then return it. It is thus forbidden for one to visit his friend’s house in order to use an item of his without his permission even on mere occasion.
One knows for certain that the owner is not particular: If one knows for certain that the owner is not particular against one using his item, then it is permitted to use it without his explicit permission. [This, however, only applies to using the item and then returning it to the owner in the same state. However, it is always forbidden to “borrow” an item of another for the sake of keeping it, or returning it in a damaged state, then it receives a more stringent ruling, as will be explained in Part 2!]
Items that people are not generally particular against being used: Those items in which it is not common at all for any people to be particular against someone using without their permission, such as items that do not receive any loss or damage at all through being used, it is permitted to use these items without permission [and then return it without any loss occurring to it]. However, those items which minorities of peoples are particular against them being used by others due to worry of damage, then it is forbidden to be used without permission even if majority of people are not particular. This applies even if one is positive that no damage will occur to the item at all, it is nevertheless forbidden to borrow it without permission.
Coming up in Part 2:
“Borrowing” with intent to keep, such as to borrow food to eat, or borrow a piece of paper.
It is forbidden to use someone else’s item without permission, unless one is certain the person is not particular against one doing so, or it is not common at all for any person to be particular, and one will return the item to the owner without any loss occurring to it.
May one use someone’s pen without permission?
Coming up in Part 2!
May one use someone’s phone without permission?
No, unless one is certain the person does not mind him using it, and using it will not cost the owner any money.
May one borrow a piece of paper from a friend without permission?
Coming up in Part 2!
May one use a hotel’s swimming pool without permission?
 Admur O.C. 14:13 and C.M. Metzia Upikadon 27 regarding borrowing Sefarim; Metzia Upikadon Halacha 4 regarding benefiting from lost object and taking fruit of friend; Gezeila Ugeneiva 10 regarding benefiting from stolen item; Sheila Vesichirus 5 that it goes without saying that this applies to other items of Reshus; Rama O.C. 14:4; Smeh C.M. 75/6; Nimukei Yosef Tzitzis 12a in name of Ritva; See Piskeiy Teshuvos 14:10
 The reason: As borrowing an item without permission [i.e. Sheila Shelo Midaas Baalim] is considered stealing and is thus forbidden. [Admur C.M. Metzia Upikadon 27 in parentheses; Sheila Vesichirus 5; Metzia Upikadon Halacha 4]
 Admur 14:13 and C.M. Metzia Upikadon 27 regarding Sefarim; M”A 14/10
 Admur Sheila Vesichirus 5; Rama C.M. 292/20 regarding Pikadon of Sefarim by Talmid Chachaim; Smeh 292/45
Contradiction from Metzia Upikadon Halacha 4: Admur states in Hilchos Metzia Upikadon Halacha 4 that one may never benefit from another person’s item without his knowledge even if one knows for certain that his friend will not mind, thus contradicting his ruling in Sheila Vesichirus 5. Perhaps, however, one can suggest that the two Halachos refer to different cases and actually compliment and qualify each other. In the case that one desires to use the item and then return it to the owner with no loss occurring to it, such as reading a Sefer, then it is permitted to do so if one knows for certain that the owner does not mind, as in essence nothing has been taken/stolen, other than its use, and on this we know the owner does not mind. This is the ruling in Sheila Vesichirus 5. However, if one desires to take some of the item and consume it, such as to “borrow” a piece of paper for writing, or “borrow” a fruit for eating, then since he is in actuality taking the physical item from the owner and this item will not be returned, then it is forbidden to do so without explicit permission of the owner even if he knows for certain that the owner does not mind. This then is the ruling in Metzia Upikadon Halacha 4. Vetzaruch Iyun!
 Admur Sheila Vesichirus 5; Shita Mekubetzes Baba Metzia 22b
 Such as Sefarim, in which the owner of the Sefarim suspects that perhaps he will spend a long time reading from it to the point that they will tear due to over usage, and therefore he is not pleased with allowing another to read from it at all. [Admur 14:10; Nimukei Yosef Tzitzis in name of Ritva]
 The reason: Although majority of people are not particular due to that chance of damage is very rare, nevertheless, we do not follow the majority in this regard to say that one can assume that this owner is not particular against it being used. [Admur ibid]
 Admur 14:13 and C.M. Metzia Upikadon 27 in parentheses regarding Sefarim
The reason: As nevertheless, the owner of the item suspects that perhaps it may get damaged, and thus if he knew that this person was using his item, it is possible that he would be particular, and it is thus found that he has borrowed without permission of the owners [Sheol Shelo Midaas], which is considered stealing, even if one causes no damage or loss to the item at all, as explained in Hilchos Metzia Upikadon 27. [Admur ibid; Metzia Upikadon ibid in parentheses]
 See Gr”a C.M. 163; Minchas Yitzchak 7/130; Piskeiy Teshuvos 14/10; See Chesed Lealafim 14/5, brought in Kaf Hachaim 14/31, for a similar defense regarding Siddurim; Lehoros Nasan 6/127-128