答案:用第三人称单数形式 is
be 动词有三种:am ， is ，are
相关用法有一个顺口溜
am is are，姐妹三朵花
I用am，you用are
is紧跟她，他，它
复数形式全用are.（eg1.we are good friends . eg2.They are good friends.）
Tom属于第三人称，所以用 is
扩展:be动词除了人称的考查，还有时态的变化。一般过去式的情况下，单数用was，复数用were。
eg1. I was surprised when I received a present.
eg2. We were happy when we received presents during birthday.
