When the time comes, you'll be able to make Windows 10 look like Windows 7
I'm pretty sure you'll be able to do that, but I can't say for sure. Windows 10 is still a work in progress, and the available Technical Preview keeps changing. So consider this an educated guess.
Let me start with the basic warning: Do not install Windows 10 on your main computer until the commercial version is released. For the time being, install it on a spare computer, a virtual machine, or not at all.
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Before you try to make Windows 10 imitate Windows 7, try it as Windows 10. Unlike Windows 8, it has a Start menu, and while it's not a Windows 7 twin, you may grow to like it.
But if it doesn't please you, you can change it.
Here's one change that I suspect I'll make right away: The search field (now powered by Cortana) is no longer part of the Start menu. Instead, it sits constantly on the taskbar, where it takes up way too much space.
You can fix that: Right-click the taskbar, select Search and pick an option. The Disabled option makes it go away entirely, but it will reappear when you bring up the Start menu.
But how do you get something like the Windows 7 Start menu While researching and writing this article, I used Windows 10 Preview Builds 9926 and 10061. Most of the Start menu configuration features found in early versions were disabled. They'll probably come back...or be replaced with better ones. So it just might be possible to configure the Windows 10 Start menu to create a reasonable facsimile.
But if you can't recreate the Windows 7 menu in the final release, and that's what you truly want, go with Classic Shell. It's a free Start menu program that I currently recommend to anyone using Windows 8. It works with the Windows 10 builds that I've tried, and the program's website lists Windows 10 among the supported operating systems. So I suspect it will work just fine.
By the way, in addition to offering a Windows 7-like Start menu, Classic Shell can also alter Windows 8 and 10's File Explorer to look and behave more like Windows 7's Windows Explorer. But File Explorer is such a clear improvement over the previous versions that I recommend you skip this feature.