Re-using and copying are two slightly different processes.
Re-using - Copies the questions from the original project and creates a new project, but retaining the data from the new project to feed back into to the original project (Merges data between the projects.)
Let's look at a scenario where this process could be useful: Imagine you are undertaking a project about 5-year development in your city. You want in the first instance to survey participants who live downtown. You also wish to question people who live outside the city or in the suburbs but you realize you need different branding (on your webpage) for the two groups of participants. In this case, you will want to ask both groups of participants the same questions, i.e. re-use them. And, you will want to store all the data in one central place, i.e. aggregate the answers from both groups.
Copying - In this scenario, the data from the new survey (the copy is stored seperately from the data for the orignial survey you copied.)
This would be useful if you were doing a pre- and post-survey. For instance, you may have a survey you run during January, and then again three months later (asking the same questions), but you want to keep the responses separate to compare and analyze any changes in the answers.
If you want to copy an entire project, go to the Settings tab on the far right of your survey page.
Click the Copy to a New Project Page button
From the Copy Project dialog box, select whether you'd like to re-use or copy the project.
Re-using a survey
Click Collect data for these questions in another project.
Type a name for the new project and click Re-use.
Copying a survey
If you want the new survey's response data to be stored separately from the original survey's data, click Create a new, separate project similar to this one from the Copy Project dialog box.
In the next dialog, edit the name for the new project and click Copy.
Your new survey is created!
Tip: In the Copy Project window, you will also see there is an option to create a copy of the project in another language. This allows you to make a copy of your survey and quickly translate it using Google translate by using a custom language link.