building(复数形式)——buildings
bridges(单数形式) ——bridge
there are not(缩略形式)——there aren't
解析：
（一）
可数名词单数变复数的常见规则如下：
1、一般情况下，词尾加“s”
如：book——books
pen——pens
ruler——rulers
2、以“s、x、ch、sh”结尾的可数名词，词尾加es
如：bus——buses
box——boxes
peach——peaches
fish——fishes
3、以“辅音+y”结尾，变“y”为“ies”
如：city——cities
baby——babies
butterfly——butterflies
4、以“f”或“fe”结尾，变“f”或“fe”为“ves”
如：leaf——leaves
knife——knives
（二）可数名词复数变单数规则，正好与单数变复数规则相反，不再赘述。
（三）常见英语词组缩写形式如下：
is not——ins't
are not——aren't
it is——it's
I am——I'm
he is——he's
she is——she's
they are——they're
they have——they've
he has——he's
she has——she's
注意，其中he is——he's，she is——she's要和he is——he's，she is——she's区别开来(表示“有”概念时，'s=has；表示“是”概念时，'s =has)
如：
He's/She's got a new bag.他/她有一个新书包。(He's = He has, She's =She has)
He's/She's a good student.他/她是个好学生。（He's = He is, She's =She is）