Question

what is your favorite subject? 是不是还可以问成“what are your favorite subjects?

Answer 1

你好
what
are
your
favorite
subjects。是指问：你最喜欢的科目有哪些。
what
is
your
favorite
subject。是指问：你最喜欢的科目是哪一个。
前一句是are做的be动词，are是复数形式，那么就是问的是那些，而不是哪个。后一句，是is，单数形式，哪一个。用复数是，subject要加s。单数不加。
这应该就是区别。希望帮到你

Answer 2

如果是同义句转换应该是what
subject
do
you
like
best?
如果是单数改为复数就是what
are
your
favorite
subjects？

Answer 3

如果是同义句转换应该是what
subject
do
you
like
best?
如果是单数改为复数就是what
are
your
favorite
subjects？