Repair/add subfloor plank patch
It's kind of hard to see the context here, but I'd guess the best way would be to get rid of all the pieces of planks and miscellaneous bits on the right side until you can get to the next joist on the right.

Then cut the existing plank(s) so that they overlap half of the joists on the left and then attach new planks to use the other half of the joists... The "seam" for the old, existing planks and the new, replacement planks will be centered over the joists.

Similar to this photo:

Except in the photo they're replacing the sub-floor around a toilet. Notice how the joists are half-exposed on either end of the cut out area. In your case, you'd fit the planks into the cut out area and screw them into the joists.

If you're not comfortable splitting the joists like that, you have a couple of other options:


You could glue and screw boards along the existing joists to provide
additional surface to attach the floor planks to. This is known as
"sistering" the joists. It's not too hard to do, but may be kind of
"over-kill" in this situation.
If you can remove the whole plank from wall-to-wall and replace it,
that may be preferable.