Build a Spaced Repetition System for Anything You Need to Remember

You read it, you understood it, you forgot it within a fortnight. Spaced repetition is not a productivity fad; it is how memory actually works. This explains the principle in two sentences and then builds you a working system: what to put in, how to write a card that works, which tool to use, and a fifteen-minute daily habit that does not fall apart.

Explain how spaced repetition works and help me set up a system to memorise {SUBJECT_MATTER} (e.g. French vocabulary / a new programming language's functions / medical terminology / historical dates). Cover:
1. The science in two sentences: what spaced repetition actually does and why it is faster than re-reading
2. How to decide what is worth putting in the system and what is not
3. How to write a good flashcard versus a bad one: three rules with examples
4. A comparison of three tools (Anki, Remnote, and a paper system) and which suits which learning context
5. A daily routine that keeps the habit under fifteen minutes and does not collapse on busy days

Keep the explanation short and the practical guidance concrete.