Hi Prithu Goswami, if you mean the u-substitution videos, this ought to be fixed very soon. The order of the videos, which had u-substitution appear after several videos where it was being referred to, was reported as "fixed" by the troubleshooters, but apparently it will still take a little while before you can see the change on the KA-site. "Fixed" in this case means the signal has been given to make the change.

If it's not about u-substitution, please send a Help request about the matter, mentioning which concept is not taught before it is being referred to, in which video.

(volunteer Star Guide)

I just started Integral Calculus today, and I am finding this issue long before the u-substitution section. For example, the first main topic is Definite Integrals Introduction. It has an article icon next to it, so I figured, okay, they want us to read first before watching a video. But it is an article consisting of only review problems! For anyone who has never seen an integral sign, especially a definite integral, they’d be lost immediately! And for someone like me, who does know a bit about calculating definite integrals but had forgotten that if part of the area is below the x-axis, it’s to be considered negative, there’s no previous lesson on this.

Then this is followed immediately by a practice with similar problems. At least the problems are based on areas learned in geometry, not calculus, so they are doable for beginners who have figured out the integral symbol.

The first video in the second subtopic under Definite Integrals Introduction refers to Reimann sums. Whoa! They haven’t been introduced yet, either (read this as “eether” when I have typed it)! I see that this is the second main topic in the course. But I suspect that the study of Integral Calculus probably needs to begin with the third main topic, Fundamental Theorem of Calculus, or maybe even the fourth one, Indefinite integrals. I’m going to experiment in skipping around until I am comfortable. But I really wish Khan Academy would put some thought into this area and straighten things out.