[2Jedismom]

Which is right?

Thank you to Richard Smith who made valuable suggestions that were essential to the completion of this project.

Thank you to Richard Smith who made valuable suggestions that was essential to the completion of this project.

[Peanut Gallery]

suggestions were


I think

[JenB]

Suggestions were. You match the "were" to the "suggestions", which is plural, not the "Richard Smith" which is singular.