1. be known as 意为“被称作”、“被认为”。

Eg：These chocolate bars are known as something else in the US, but I can't   remember what.

    这种巧克力棒在美国有别的叫法，但我记不起来叫什么了。

       2. be known for 意为“因…… 而著名”。

Eg：He was known for his frankness.

    他以坦率而著称。

    3. be known to意为“为……所知”。

Eg：He is known to the police because of his previous criminal record.

    他因以前的犯罪记录而为警察所知。