1. Normally, the plural of "deer" is deer", but the plural "deers" does exist.
I would use "deer" there.
2. You don't have to return the books until you (come to) see me next time. The words in parentheses can be omitted unless there is a reason for them to be there.
3. The third sentence is fine. This is sometimes called the double genitive, but it is correct in English. One could also say "His garden is beautiful."
The American Heritage® Dictionary of the English Language: Fourth Edition. 2000. double genitive NOUN:A phrasal construction in English in which possession is indicated by the word of followed by the possessive form of a noun or pronoun, as in a relative of mine or a friend of Pat's. Also called double possessive. See Usage Note at of.
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