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Borrowing someone’s Shofar without permission:
It is permitted to use someone’s Shofar without permission in order to blow it to fulfill the Mitzvah. One is to say a blessing over the blowing in such a case. [However it is forbidden to remove the Shofar from the room if one did not receive permission to do so, and the above allowance is only to use the Shofar in the same room that it was left. If the owner explicitly states that he does not give permission for his Shofar to be used then it is forbidden to use it.]
How many sounds may one blow with a Shofar borrowed without permission?
One may blow the 100 sounds that are accustomed to be heard even though one fulfills his obligation with 30 sounds.
May one borrow a Shofar without permission for the sake of blowing for women or children?
May one borrow a Shofar without permission for the sake of practicing?
If one borrowed a Shofar and damaged it must he pay the owner for the damages?
Negligence: If the Shofar was damaged due to negligence then the borrower is obligated to pay for the damages.
Process of blowing: If the Shofar was damaged in the process of using it, such as it cracked, then if he borrowed it with permission he is exempt from damages. If however he borrowed it without permission [as stated above that it is allowed] then some Poskim rule he is liable for all damages. Others however rule he is exempt from damages just like one who borrowed it with permission.
 586/5; M”A 586/4; Elya Raba 586/7; Chayeh Adam 140/18; see also 14/9 regarding Tallis [permitted]; 14/12 regarding Tefillin [permitted]; 14/13 regarding Sefarim [forbidden]; Admur Choshen Mishpat Shiela Usichirus 5 regarding Sefarim that if minority of people are Makpid is forbidden, if knows for certain no one is Makpid is permitted
 The reason: One does not need to ask permission to use someone else’s Shofar if he intends to blow for the Mitzvah as one can assume the person does not mind a Mitzvah being done with his object without incurring him any loss. [ibid; 14/9 regarding Tzitzis]
Other Opinions: See Piskeiy Teshuvos 586 footnote 6 that perhaps today due to extra cleanliness people are more careful that other mouths do not touch the Shofar and hence it would be forbidden to use a Shofar unless the person received explicit permission. See Aruch Hashulchan 14/11
 Shaareiy Tziyon 586/20; Kaf Hachaim 586/23; Admur 14/10 regarding Tallis; 14/12 regarding Tefillin; M”A 649/16 regarding Lulav; Piskeiy Teshuvos 586 footnote 6
 M”E 586/5; M”B 586/9; Kaf Hachaim 586/23
 Hisorerus Teshuvah 1/177; Igros Moshe 2/106; Piskeiy Teshuvos 586/3
 Chayeh Adam 140/18; Kaf Hachaim ibid; Piskeiy Teshuvos ibid
 Choshen Mishpat 291/1 [regarding a Shomer Chinam, and certainly this would apply to a borrower in this case even if one holds according to the Machaneh Efraim]
 Birkeiy Yosef 586/10 ; Halachos Ketanos 1/79; Shulchan Gavoa 586/8; Kaf Hachaim 586/25
The reason: As a borrower [Shoel] is only liable for all damages if the damages did not occur in the process of the normal use of the object. [Choshen Mishpat 340/1]
 Birkeiy Yosef 586/10 in name of Mahriy Malko 98; Shaareiy Teshuvah 586/2 ; See Kaf Hachaim 14/18 and 586/25
 As this has a status of “Shoel Shelo Midaas” in which case he is liable in all cases.
 Halachos Ketanos 1/79; Shulchan Gavoa 586/8; Beir Heiytiv 586/4; Sdei Chemed Daled Minim 9
 Orin Telisain 97 based on M”A 586/6 as he has a status of a borrower which is liable for even Onsin. [Choshen Mishpat 340/1]; See Sdei Chemed ibid; Piskeiy Teshuvos ibid footnote 9
 Machaneh Efraim Nedarim 24 as the borrower receives no monetary benefit from the use of the Shofar and he thus does not have the status of a borrower in this regard.