What kind of Shomer is the Pasuk referring to?
Rashi (on Pasuk 9), Ramban, Rashbam (both citing Bava Kama, 94b) and Targum Yonasan: It is referring to a Shomer Chinam, whom the Torah renders not liable for theft (and loss).
How do we know that this Parshah is referring to a Shomer Chinam and the following Parshah (Pasuk 9-12) to a Shomer Sachar, (whom the Torah declares liable for theft (and loss)?
Ramban and Seforno: We know that from the fact that the Torah changes from money or vessels (which people tend to safe-keep for frees, to animals, which they generally place under in the charge of a shepherd, whom they have to pay.
Why does the Torah render a Shomer Chinam Patur from Geneivah va'Aveidah, and a Shomer Sachar, Chayav?
Rashbam: Because, since the former is safekeeping inanimate objects, 1 the owner knows that the Shomer will place them in his house together with his own belongings, which he guards with care; whereas the latter, he places in the field, 2 and he expects him to keep a watchful eye on them.
Bearing in mind that the following Pesukim are speaking where the article was not stolen (See Sifsei Chachamim) explains, how can the Pasuk say "ve'Gunav mi'Beis ha'Ish"?
Rashi: The Pasuk is referring to the Shomer's claim, not to what actually happened.
Ramban: The Pasuk is indeed speaking where the article is stolen, and it then draws a distinction between whether the Ganav is found or not. 1
What is considered giving to guard?
Moshav Zekenim: The owner must say 'here, guard this for me.' If he said only 'keep your eyes on it', he is not a Shomer, and he is exempt for [passive] negligence. If he gave it to another Shomer, and we know that Ones occurred, he is exempt; the Halachah does not follow Rava, who says that the owner can say 'I do not want my item in another's hands.' 1 If one deposited with a woman in front of her husband, he is liable, as if it was deposited with him.
Moshav Zekenim: One may not do so l'Chatchilah. One may lend Seforim to others (who will not ruin them) without the owner's permission, for one is pleased that his property be used for Mitzvos.