Depending on what you are trying to accomplish, you could use:
There are several different tax software compatible with Linux. You can use Tax Act. Is another software that people use when the Linux is not available.
Linux is not illegal. Linux is a free, public license software modeled on Unix.
Completely free. If someone sells you Linux, it is legitimate, however, it will not be for a license to use the software. The GPL is not that kind of license. However, it is still free as in freedom, not free as in beer, meaning it is entirely valid under the GPL to sell Linux.
Turbo Tax and Linux tax software both do your taxes very easily and with a good user friendly look. The best tax software to use though is Turbo Tax because it is superior to Linux.
ext4
By configuring the bootloader to use Windows as the default menu option instead of Linux. Refer to documentation on how to do this.
Yes, you can use Linux software commercially, depending on the licence issued by the software developer. A lot is free entirely, some are free for home (non-commercial) use only, whilst some come with a commercial licence fee to pay.
Some people refers to Linux-based systems as "GNU/Linux" if they use software written fr the GNU operating system.
Dreamworks uses a proprietary software that runs on the Linux platform for most of their animations.
Open source (free) software (Operating Systems based on the Linux kernel: Ubuntu, Linux Mint (to name my two favourites) are, in my opinion, the best OS software to use. Both come with many free application software programs for the downloading off the internet.
Most applications are available at your distribution's repositories, however depending on the kind of distribution, you may have the option to use user-created repositories that allow you to use applications or dependencies that offer features the official packages do not have, or that the official packages are not offered due to legal issues (some distributions have a strict policy against including non-free/proprietary software in their repositories).
ISO Master and K3b.