在给定的英文文章中找到所有动词并列出它们。

以下是一篇英文文章的样本，我们希望您在其中查找动词：
In the early days of American animation, animated characters were silent, as the technology to synchronize sound with the animation did not exist yet. In 1928, Walt Disney introduced the first cartoon with synchronized sound, Steamboat Willie, which starred Mickey Mouse.
动词：

- were (be)
- did (do)
- exist
- introduced (introduce)
- starred (star)