Chuchu is a pet dog. His master is an office worker. Master comes back. He is happy. Master got married. Master gave food to the dog. Is it good? Yes, master. It's delicious.

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Q1: Can we use "Master"? In the sentences, "Master' seemed to be used as a proper noun, right?

Q2: In the second sentence from the bottom, the dog said, "Yes, master."

In the last expression, 'master' is used in the vocative case. So we don't put 'the' before master. Is it right?
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When did master lose Chuchu?
- Master lose Chuchu when they went out for a walk.

(Can we use 'master' instead of "Master" or "the master"?)

It's very awkward to use a title in place of someone's name. If you put a name after the first instance of "master" and then use the name from then on, it will read better.

The sentences are all very short, and when reading, it sounds rather "choppy." This would be smoother if you combined the short sentences ... like this:

Chuchu is a pet dog whose master, Tom, is an office worker. Tom comes home and is happy because he recently got married. Tom gave food to Chuchu. "Is it good?" Tom asked. "Yes, master, it's delicious," Chuchu seemed to say.

Q1: In the sentences you provided, "Master" is indeed used as a proper noun. It is capitalized and refers to the specific person who is the owner of the pet dog.

Q2: In the expression "Yes, master," the word "master" is indeed used in the vocative case, as a form of address to refer to the owner. In this case, it is more common to use "master" without the definite article "the" before it. However, in some contexts, it is also acceptable to use "the master" when referring to someone as a title or position, emphasizing their role as the owner or leader.

Regarding the additional question, it is grammatically acceptable to use "master" instead of "Master" or "the master" in the sentence "Master lost Chuchu when they went out for a walk." However, using "Master" or "the master" would be more common and clearer in indicating that it refers to the specific person who is the owner of the dog.