be careful 中充当谓语成份,如:
Be careful of his feelings.关心一下他的情绪,
Be careful with the glasses,小心玻璃杯.
Be 又是动词又是介词,后面可以跟形容词/名词/动词不定式.
The test was yesterday.
昨天进行了测验
The food is on the table.
食物在桌上
be to do sth 可表将来时态.
.