As a rule of thumb, don't totally leave a topic until you know "how", and understand the "why". Add me at Skype, my Skype id is- akshat.delhi
M. Sc. & Phd.
Experience: 20 years
Let us strive to learn for understanding of mathematical concepts and procedures, the "why" something works, and not only the "how". The "how" something works is often called procedural understanding: you know how to work long division, or the procedure of fraction addition or fraction division, for example. It is often possible to learn the "how" mechanically without understanding why it works. Procedures learned this way are often forgotten very easily. The relationship between the "how" and the "why" - or between procedures and concepts - is complex. One doesn't always come totally before the other, and it also varies from child to child. And, conceptual and procedural understanding actually help each other: conceptual knowledge (understanding the "why") is important for the development of procedural fluency, while fluent procedural knowledge supports the development of further understanding and learning. Try alternating the instruction: learn how to add fractions, and practice on it. Then understand why it works. Go back to some practice. Back and forth. Sooner or later it should 'stick' - but it might be next year instead of this one, or after 6 months instead of in this month.