When is an item rendered as learned?
What does it mean? Hoe many times I need to remember something to make it counted as LEARNED?

You need to get it right with an interval between repetitions of more than 12 hours.

So getting an item right twice, with an interval of at least 12 hours in between, is enough to consider the item 'Learned', no matter how many times I got the item wrong before that?

When is an item 'forgotten'?

Does this information appear somewhere in the FAQ?

But, of course, even 'learned' items come back onto reviews at some point. Even if something is learned, that doesn't mean it will stay 'learned'.

Interesting question though, I'd like to know the answer too!

It's not that the actual interval between repetitions needs to have been more than twelve hours, but that the next scheduled repetition interval is more than twelve hours. So if it was scheduled for six hours and then you get it right after six hours, you can have crossed the learned threshold because your next interval might be thirteen hours or so.

It counts as unlearned whenever you drop back below a twelve-hour scheduled interval.

The "learned" designation is only used when adding up your progress numbers, really. You can imagine that there are a few items that are at an interval of more than twelve hours and less than a week, which maybe you wouldn't consider "learned" in the sense of "are you solid on this yet?", but the number of those is still very small compared to your totals, since almost all your items are scheduled at long intervals at any point.

Nick, thank you for the clarification.

I have a related question. The "So how does the spaced repetition scheduling work, exactly?" section of the help page explains what happens when an item is graded 1 to 4. What happens if I am not using the grading option? Does a mistake always counts as a 1 (don't know), or does it sometimes count as a 2 (so-so)?
Does a correct answer always count as a 3 (got it)? Does the yellow-orange indicator on the iOS app corresponds to (so-so)? Is there something similar on the web site?

Thanks again!

A mistake is counted as a 1 (red), although if you write a few strokes wrong sometimes you get a 2 (yellow). If you miss one character in a word with three or more characters, the word-level item can be graded as a yellow 2 while the wrong character gets a red 1. If you don't grade anything, it's a green 3. The iOS app and website use the same grading and colors.

Can you get yellow on a single character on the website? I don't recall seeing it.

I guess that's only for holding down to trigger a few strokes of the stroke order animation. You can also turn on the grading buttons or use the keyboard shortcuts and manually mark it.