is am are 用在一般现在时的句子中
were was 用在一般过去式的句子中
being 可以和别的助动词构成别的时态,例如will be beding.../have been being...
been to 用在have/has been to中 意为曾经到过.
to be一般用在非谓语中,例如, there are so many things to be done.