You can easily reply to a certain message with names prepopulated by hovering your mouse to the message, and click Reply. [Reply aid=* Number to=*Numbe] describes the user you are replying.
Or, you can always select someone at the [TO] field to reply.
If you wish to quote certain message, click [Quote] in the menu displayed when you hover your mouse. The original message will be displayed in the message field. Also, you can add/edit the message to add more context.
Highlight the part where you want to quote, and select [Quote in message].
Please keep in mind that [Quote] [/Quote] is a special protocol used in ChatWork, please do not edit this.