可以教我這句英文嗎?

Change the nouns into plural form, Make other necessary changes and rewrite the sentences.解作：更改的名詞變成複數形式，進行其他必要的修改和重寫的句子。

bread (面包)是不可數的名詞，所以不用加s ，piece是可數的名詞所以加s

a代表一台、一個等等，而some代表一些

所以是some computers

i think your book's answer is not correct too.
if its a plural for your question the answer should be: "these breads are delicious"
instead "these pieces of bread is delicious" piece of bread is a word used in single form sentence piece of bread= 一個麵包 so the answer of your book should be wrong. you can ask for your teacher.
hope this can help you with your problem:)

2013-02-26 20:03:38 補充：
to the problem that why "bread" don't have to add a 's" this is because you already used the 量詞 and added "s" in their so you don't have to add an "s" in "bread" this is a rule of english, there are no reasons.

2013-02-26 20:05:56 補充：
"there is are computer" is already wrong. you cannot put is and are together in one sentence. and "a" have to change into "some" because "a" and "some" is a 量詞 you have to change the 量詞 also in the sentence
a= single
some=plural
so that's why you will have to change the "a" also!

You have to understand that by changing a sentence from singular to plural or the other way around, you will have to change every word that has a singular/plural form.

In the first case,
this-these
piece-pieces
is-are
(bread is uncountable and can't be changes into 'breads')


Similarly for your computer sentence,
is-are
a-some
computer-computers

As 'a' is talking about a single object like a tree, a dog...
You will have to change it's form
There is no definite plural form for 'a',
depending on whether the object is countable, you can use 'some'/'a few'/...

*You cannot change a-are because 'are' is the plural form for 'is' and has no relation with 'a'


I think you need to find and English tutor, because singular/plural are just basics in the English grammar and I'd be willing to help.