英文grammer

My dog is nice.Their dog is good too.(theirs)
Ans. My dog and theirs are good.
(theirs = their dog)
My dog and theirs are good. = My dog and their dog are good.

their = 他們的 ...
theirs = 他們的東西


My sister is helpful.His sister is helpful too.(his)(possessive pronouns)
Ans. My sister and his are helpful.
(his = his sister)
My sister and his are helpful. = My sister and his sister are helpful.

his (unstressed 輕音) = 他的...
his (stressed 重音) = 他的東西

My dog is nice.Their dog is good too.(theirs)

Their dog = theirs (用 theirs 後面是不需要加上名詞的，己將名詞包括了)
所以，這句是將 theirs 代替 their dog 而變成：
My dog is nice. Theirs is good too.

2012-03-19 13:33:51 補充：
my dog = mine
our dog = ours
your dog = yours
thier dog = theirs
his dog = his
her dog = hers
its dog = its

但有一件事要留意，就是名詞在較早的地方要表達了，交流的相方都清楚在講甚麼。

2012-03-19 13:36:19 補充：
My sister is helpful.His sister is helpful too.(his)(possessive pronouns)

His sister = his
所以代入 possessive pronouns 後，便變成：
My sister is helpful. His is helpful too.

也可以用 Both 將兩句連接：
Both my sister and his are helpful.

2012-03-19 13:37:49 補充：
The sister is his and she is helpful.得唔得咖?
這樣寫，she 便變了 his sister 。在意思上便和原來的兩個句子的意思不一樣。所以不正確。