This question has come up rather frequently lately so decided to construct a tip from it. The problem arises when a Gmail user wants to forward a message from earlier in a Gmail conversation (group of emails about the same subject…see illustration and video).
When you click a conversation, Google only displays the most recent message. If you click forward, then all the messages in the conversation go along for the ride (unless you edit the message you are sending…see video). To forward just the first message (or any specific message in the conversation), first open the message by clicking on it, then click forward.
This makes a lot more sense once you view the video then try the technique yourself.