1. If one is thinking in developing software similar to iKnow, what are the main principles? and what are the main differences between iKnow and SRS software like Anki, Mnemosyne, or SuperMemo?

2. What SRS has these features:

a) Typing the answer.
b) Multi choice (automatically)
c) Reversing (e.g. if I enter Q: mizu A: water, it will automatically generate Q: water A: mizu)

Anki has an option for typing the answer and card reversal. No option for multiple choice, although I don't think many people on here are a fan of iKnow's multi choice system.

If you are going to develop software for commercial reasons, keep in mind that iKnow and Anki are both extremely strong products (Anki for SRS, iKnow for content) and that they are free to use (funded by donations and advertising) and customizable, making them a no-brainer for most people.

There are also a number of other strong products on the market which cover other niches (supermemo, mnemosyne, iflash, mental case, and this site to name a few).

I would suggest that you clearly identify what it is you don't like about the other products and then decide what your unique contribution would be.

In relation to iKnow, I think it's strengths are accessibility (for non-technical or computer literate types), strong multimedia content and social networking facilities. It's major weakness (for me) is poorly conceived long term review and over-reliance on inflexible multiple choice type questioning.

Anki's strengths (for me) are it's solid SRS functionality, incredible flexibility and great support. It's weaknesses (for me) are an unrefined user interface and poor card management (compared to something like iFlash, for example) however this is easily forgivable under the weight of it's strengths.

I think it's vital that you get these programs, do some testing and research and get some real focus before you start coding anything (even if this project is non-commercial and just for fun). Sure, it's a lot of work, but it's the only way you'll be able to answer your first question and create a killer product.

PS The answer to your second question is none (afaik), but then most serious students don't really mind the lack of multiple choice and Anki covers everything else. Might be useful for people sitting multiple choice exams to test against past papers though, so I wouldn't necessarily throw away the idea completely.

EDIT: Oh, and good luck!

You forgot to point that some like anki and mnemosyne are free also in another sense. They are free like the libre in spanish or livre in portuguese. You have liberty to change and redistribute them.

Despite always being connected to the net, I now use Anki rather than iKnow. The multiple choice thing is -not- a help and iKnow spends a lot more time per word than is actually necessary to learn it. I spend a fraction of the time I used to on iKnow, and I learn just as much and keep the old stuff fresh, too.