Before installing the new floor, I pulled up the carpeting and all. I found the subfloor near the sliding glass door to be in bad shape. Once the carpet came up, I can feel a lot of air coming thru the bottom of the door. Not only that but last week I was in the crawl space and noticed light coming in from outside (a hole in the outside wall or something). I now realize that that light I saw coming thru was in the same place as this deteriorated subfloor. So I ask myself if I have a subfloor problem only, or is the door shifting, or what?

http://i1380.photobucket.com/albums/ah185/sputterbug10/floor%20by%20sliding%20glass%20door_zpsosrdizro.jpg

http://i1380.photobucket.com/albums/ah185/sputterbug10/crawlspace%20light%20coming%20thru_zpslsucsgjp.jpg

It certainly is not uncommon to have water damage near the side of a sliding glass door. There are weep holes that could get plugged up with dirt, pet hair or other things. If the intersection of the door jambs and the sill has caulking that leaks, it will allow rot to start. Being covered with carpet will hide water stains or delamination of the plywood and you will have a surprise when the carpet is removed. You may need to take a section out and replace with new. This could involve some new framing, whether the supporting joists have deteriorated or not.
Now would be the time to get a free estimate from your floor contractor.
Before you install a new floor, after you have checked for leaks, gently spray the door on the outside of the building with a hose to see if there are remaining leaks.

Looks like the top edge of the rim joist has rotted too. Is the first picture the outside ground or patio that abuts the house? If so it may not have been flashed or graded properly. Good place for water leaks if opening is not flashed before the door unit is installed.

I looked again and the first picture is inside the house? Comment about water leaks and door flashing still true.

The picture is from the inside. After cutting into the floor to pry it out, I found out that the previous owners laid this crumbling type wood subfloor on top of 1/2" plywood. The plywood goes under the wall so I'm not even sure how to lay the new subfloor except to cut a straight light around the perimeter with a circular saw and then butt the new plywood against the old. Problem is I bought 3/4" plywood so it won't match. What a nightmare. I don't think there is flashing. the deck outside the sliding door is pulling away from the house and I don't see flashing, but it's difficult to see anything to be honest.
Are the weep holes along the bottom? I don't see any.

Do you have a picture from the outside? Is the deck attached to the house? If so, then the deck ledger board is attached to the rim joist. The rim joist is what you are seeing from inside the crawl space....the light is coming through the top section of rim joist which appears to be rotting. If the rim joist is deteriorating then the deck ledger board screws maybe losing their grip. How high is the deck off the ground? Don't want it to fail and fall too far. It would be a good idea to find the water intrusion points and fix them and get the deck taken care of before working inside on the flooring otherwise the water intrusion will ruin your inside work. Is the entire floor covered with what looks like particle board? Plywood is 1/2", particle board thickness is ? What type of finish flooring are you planning on?

One half-inch plywood was the minimum sub-floor in the early 1980's. If you get a sheet of that thickness for the sub-floor, you can use it for the underlayment and be close to the height of the adjacent room. Where you have a joint in the sub-floor you will need to either end on a joist or add blocking to support the intersection of the old and new.
You may want to order a new door, one with two layers of glass, if you have an old one. This will make the repair a little easier. Remember that when no more water leaks in there will be no more rotting. You don't have to replace all the fungus damaged wood. Just add some wood that you can glue and nail to be secure.
Working on an old house can be interesting, not knowing what you are going to find.

So I have pulled up all the flooring - what a job. Here are pictures of what's going on. I have no clue how to fix this. Any thoughts? The board that the floor joists butt up to is rotted. The deck outside the sliding glass door is not up against the house like I thought it was.

http://i1380.photobucket.com/albums/ah185/sputterbug10/rotted%20plywood%20inside%20under%20sliding%20glass%20door_zpsrklszn45.jpg

http://i1380.photobucket.com/albums/ah185/sputterbug10/board%20under%20sliding%20glass%20door%20outside_zpsuqnwhdhk.jpg

You need to fix the exterior flashing that goes under the door and protects the building from water intrusion before anything. There is no siding under the door, but you still need flashing to keep water away from the framing. The deck looks like it is or will be rotting at the joints between the planks. Water needs to drain without any trash in the gaps between planks. This needs to be cleaned out regularly, otherwise the wood will not dry enough between rains. The longer it remains wet, over 19% moisture content, the longer it has a chance to rot. The trash keeps it wet between rains. You probably need to remove some of the deck.
Back to the inside. You can scab another 2X to the rim joist that is rotted, as long as it has dried out. This will give you a place to nail the sub-floor when you patch.
Of course you could remove it , also, if it is accessible from the outside, under the SGD, which you probably have to remove to flash completely.
Once the framing is installed to make a joint in the new/old sub-floor joint, nail with 8d nails every foot. You may want to use sub-floor adhesive to help fasten the sub-floor down White glue is no good for this.

That rotted rim joist board needs to be taken care of. If you're not comfortable doing this type of work it would be best to call in a builder/carpenter for an assessment. Depending on the extent of the rot you may be able to piece meal it. You also need to determine the source of the water leak and fix it.

I know I'm in for a world of hurt money-wise. I realize I'm gotting off of the hardwood flooring subject (thank you for not telling me to get lost), but does this look like termites damage?...or just rotted board. Ive got a list of construction/remodeling contractors to call. Too bad my ex-husband and I dont get along so well; he's a successful remodeler! He's also successful at hiding his money so as not to pay much child support; thus the reason I do as much as I can myself (with the help of a brother). You guys are very helpful. This is a great forum!

Any evidence of termites? Mud tunnels on the foundation walls? Any wings? Any carpenter ants? You need a good lawyer to find the ex's hidden money....show the judge that he is hiding money to avoid paying his fair share of child support and he'll be in a world of financial hurt.....my mother went through this with dear ole dad so I am sympathetic. How's your brother's knowledge on this stuff?

If you are worried about insects, you can treat the wood before you flash the door.
Either copper green, which may not be available where you live or Bora-Care.
Borax treatment will kill fungus, too.

You may find termite frass after they have been working for a while. They will be happy inside the wood without making any holes. They usually like to have an invitation of damp wood. Water penetration into the house will provide a perfect starting point. The termites crawl over every surface looking for a home after they lose their wings. Termites don't have a brain to look for a home, hire someone who knows construction if you do not have experience.