在以下句子中找出语法错误，并给出正确的句子。
a. Me and my friend is going to the park.
b. The movie was good, but I seen better.
c. They was happy to see us.
a. Corrected sentence: My friend and I are going to the park. 
b. Corrected sentence: The movie was good, but I've seen better. 
c. Corrected sentence: They were happy to see us.