A Shomer Sachar is Patur for Onsim and Chayav for Geneivah and Aveidah, a Sho'el is Chayav for Onsim and it is a Kal va'Chomer that he is also Chayav for Geneivah and Aveidah.
A Shomer Sachar pays Kefel if he claims the object he was guarding was stolen by armed bandits and witnesses testify that the Shomer Sachar stole it. (1)
It is a Machlokes if the rule of Dayo applies to a Kal va'Chomer and we may not learn out a Din from unless that Din also applies to the Halachah that we are learning it out from.
If a Shomer is negligent and the owner is with him in the Melachah it is a Machlokes if he is Chayav or Patur.
If someone borrows a cow and he also borrows or rents the services of the owner along with the cow or he borrows or rents the services of the owner and than he borrows the cow if the cow dies he is Patur.
If someone rents or borrows a cow and he borrows or rents the service of the owner at the same time even if the owner is doing a Melachah somewhere else if the cow dies he is Patur. (2)
It is a Machlokes between R. Yehudah and R. Meir whether a renter has the Din of a Shomer Chinam or a Shomer Sachar.
If the owner is with the Shomer at the time of that he received the item even though he was not with him at the time of the Ones he is Patur.
If the owner was with the Shomer at the time of the Ones but he was not with him at the time he receives the item even if the owner is plowing right on top of him the Shomer is Chayav.
A BIT MORE
1. This is in accordance with the opinion that an armed bandit is a Ganav, however according to the opinion that an armed and it is a Gazlan there is no Chiyuv Kefel.
2. Even a renter is Patur if the owner is working for him at the time of the rental and even if the owner is not working with the cow that he borrowed and instead he is doing some other work for the owner he is Patur.
The Gemara states the second Lashon that an armed robber is a Gazlan and therefore there is no Chiyuv Kefel and consequently a Shomer Sachar never pays Kefel. The Tosfos Shantz asks that a Shomer Sachar pays Kefel if he claims that the item was stolen b'Ones. The Ri answers in Bava Kama that a Shomer Sachar is Chayav for Geneivah even if it was stolen b'Ones with the exception of armed robbery which is in the category of Shevayah. The reason why a Shomer Sachar is Chayav for Geneivah even though it occurred by way of Ones is because every Geneivah is close to being an Ones and even so he is Chayav.
SHE'EILAH WITH THE BA'ALIM
If someone borrows a cow and the lender is with the borrower in his Melachah at the time that he did Meshichah on the borrowed item even if the item is stolen or lost because of his negligence he is Patur. It makes no difference if he was with him by way of She'eilah that he lent his services to do Melachah for him or if he rented himself out to do Melachah for him. It also makes no difference if he was with him for the Melachah that was being done with the borrowed item or for any other Melachah. Even if he asked him to give him a drink of water and while he was pouring a drink he borrowed his cow it is considered a She'eilah with the owner. But if he did Meshichah on the item first and than he poured him a drink of water it is not a She'eilah with the owner. (Shulchan Aruch CM 346:1)