1. be familiar with表示“（某人）对（某事）是精通的、熟悉的”， 句子的主语总是表示人的名词或人称代词，介词with后接表示事物的名词。如：

Most of us are familiar with the pop star.

John was very familiar with this kind of situation.

2. be familiar to“（某事）对（某人）是熟悉的”， 句子的主语总是事物，介词to后接表示人的名词或人称代词。如：

The pop star is familiar to most of us.

This kind of situation was all too familiar to John. (=very familiar)