Up at the top of the page, there's a little bell icon (it will probably have a number next to it a lot of the time) which indicates alerts. A lot of these will be reactions (i.e. likes, etc.) by other members to previous posts, but if you read the messages, it will tell you what's happened.
The ones you probably care about are someone "quoted your post" or someone "mentioned you in a post" which if you click the link will take you right to that post. This will save you considerable amount of time so you don't have to navigate the threads page by page.
If you want you can further configure the alerts in your profile. Click on your profile up at the top and select Preferences from the drop-down. You can turn off the alerts for reactions which really helps to keep it clean. If you're mainly here just to respond to questions, it will probably help you make better use of your time.
Edit: If what you're describing is as
@Shady Attorney states below, you can click the Quote button below a post to add it to multi-quote. Once you've done that to any posts you want to respond to, scroll to the bottom of the page below the text box that says "Write your reply..." and click the button that says "Insert Quotes..." (it will only exist if you have posts selected to quote) which will bring up a pop-up window which shows all of the posts you want to quote, will allow you to rearrange the order, or remove any that you no longer wish to quote. Clicking the "Quote Messages" button in that pop-up window will add them to the text field.
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