Hi, I'm in trouble when extruding a top tale fin. Subdivision modifier divide it in two along Y axis. Why your video's sub...

You need to turn on the clipping option on the mirror modifier, so the mirrored part stays glued.

I solved myself after asking. Of course I had turned on clipping because I'd had done as the video did.When extruding or insetting, my blender makes face on the side which disturbs the modifier, so I deleted each face. I don't know the difference between my blender and your blender.

Come on, I speak Portuguese and depend on google for English. I had not understood his doubt and I had not realized it was aimed at Jonathan Williamson. But, trying again to help, you must have made an extrude (E), which ends up creating the inner face that you deleted. In the video Jonathan made an inset (I), which did not create the problematic face, and then moved it. I hope I have helped.

Again I did not understand, but I saw in time: "When extruding or insetting ...". In inset face turn off the "boundary" option and just move the face up, it will work!