Twine is based on a GPL licence. You are free to download, modify and publish derivatives - even commercially.

The basics are very easy to grasp, and you can have a basic story up in minutes.

Using the different macros correctly can allow you to make simple linear RPGs or the more complicated decision-based RPGs. Not true to the CON listed below, you can do so much more than just interactive fiction. If you don't know what your doing, just tell the nice people on the Discord server what you're trying to do, and they will gladly help. Also, once you can figure your way around the more complicated code, you can start adding visuals to your game as well.

Twine has a large website containing every macro for the Harlowe story format, I don't use the others so I don't know about them, as well as a description of how they work coupled with an example. The website makes coding with twine even easier.

The Twine community is active and full of life. As well, the Twine Discord Server is full of experts who will gladly help anyone who doesn't understand how Twine works.