There are two ways you can respond to a post made by another member and participate in a thread.
1. You can contribute
Contributing means that you want to add a new thought or idea to a thread. When you see the option to "Share your perspective..." you can type your response and contribute to the thread.
2. You can comment
The option to comment activates after a member has made a contribution in a thread. Comments are useful when you want to address a particular contribution - like adding something, showing you agree, or thanking someone for their contribution.
Comments are nested under the contribution. Many can be added, one on top of the other.