Baby robin

I found two baby robins on the ground under a pine tree, one is dead the other was still breathing but not sitting up or opening his eyes very wide, the nest was torn apart, there is another nest that we watched the babies from birth and they just flew...

from the nest, the mother always lays another batch of eggs in that nest, can I put the baby robin in that nest and will the other mother take care of it or not

No. The mother will only take care of its own eggs, unless by some freak of nature something happens, and the bird is suddenly willing to take care of the eggs. You don't know exactly which positions the mother bird had the eggs in before the nest was disturbed, and there's a good chance that they were moved when all hell broke loose for the birds. Once robin eggs have been touched by people, the birds will never go near them again. You could throw them all in the trash, or if it makes you feel better (I'd do this too) bury them.

The bird is too young to stand or open it's eyes but can be saved. Call Aark for recommendations (215) 249-1938. Take the bird to a local rehab center ASAP. Search the internet for Wildlife Rehabilitation and your state.

IN THE MEANTIME:
Feed DRY CAT FOOD moistened with water until mushy- a little tiny plastic fork- like for appetizers or something of the sort to push it into his mouth. 3 MM sized scoops every hour. Do not feed it the wrong food which is worse than no food.

If it won't open it's mouth, try to brush the underside of it's beak with your finger or, you can take your thumb and middle finger on either side of the corners of his mouth and use the nail of your forefinger to split his mouth open then put the food into his throat- you'll need to push it down, don't just put it in his mouth.

Keep him warm, an incubator is ideal but you likely don't have one, keep him warm any other way you can. Don't wet him of course. Put him in a small container and surround him with scrunched up tissues to keep his legs from splaying, and heat in, in addition to other heat source.

All I know is I have been watching the complete process on the windowsill atop our front door. I cannot easily see into the nest and only sneak a peek if the mother is off the nest, with a mirror reflecting inside the nest so I never get to see them for more than a second or two. It's been magical, and enjoyable, and we saw 4 eggs turn into 4 lovely little creatures. Last week, however, when I opened the front door I was shocked, and very upset to see two of the babies down on the ground. One was most definitely dead and the other was either dead or close to it. I was too upset to investigate and turned the job over to my husband. He never said anything about it, as he was completely upset too. the mother robin is seemingly taking care of the other two birds that are in the nest and I wish someone could tell me how and/or why those two birds were not in the nest. I do not believe they simply fell from the nest. Can someone please tell me -- does the mother ever toss out a robin if she suspects, perhaps, it is ill, mal-formed, whatever? I just wish I knew why those other two little babies did not get as lucky as their siblings. I could just cry over discovering those tiny little birds down on my doormat.