Big dogs are the easiest to train.
Contrary to popular myths, it is much better to do it with an adult dog, the personality is already developed.
I have 2 rotweiller, a bitch and a male.
The male when i got him was a little on the young side. Just under a year old but he was already probably over 40Kg.
The most important thing u need to is establish who is boss.
Contrary to the fuking treehuggers a choke chain is the most effective method.
For starters most important word for the dog to know is NO.
Get a good choke chain, make sure it will not snap. (otherwise u are in a world of hurt).
Put something like a steak on the ground, when the dog, attempt to eat the steak, yell NO, if he does not listen, choke him with the chain.
With my dog, he grabbed the steak and would not let go. So you choke him with the chain till he almost passes out, and he will release the steak.
While you are choking him, yell NO NO NO repeatedly, so that way it is firmly fixed in his mind what NO means.
Ussualy you should not need to do it more then 5 times in the entire dog's lifespan (if you have to do it more, you will ruin the dog)
One he understands NO, you can teach him how to sit, and what other crap u want to do.
With my dogs, i just teach them a few commands, sit, no and up.
Up means he is allowed to kill what ever is inside the yard. If he attempts to go outside the yard, i just yell NO.
If another dog enters the yard, and i tell him to sit, he will start to make crying noises begging me to say UP, meaning he can go after the other dog.
He has killed probably atleast 5 cats. Not much if any blood, he just grabs the middle of the cat, and crushes it. When u yell NO he releases it and the cat flops arround on the ground for a while.
He also killed a goat (approx 25-30Kg). He is pretty smart. He watched the goat, but meanwhile was pretending that he was watching something else. When the goat stuck his head thru the slats in the fence, he sprung about 5 meters, grabbed the back of the goats neck. And then pulled the goat thru the slats in the fence. 1 bite and that is all she wrote, Mr Goat. When he released the goats neck, the goats just flopped around on the ground.
Another problem that i had with the bitch. Is she got smart with me with the choke chain. When i let her off, for off leash training, she thought she could run away and hide under the house, where i can not get at her. (I yelled COME, the fuking dog did not come, instead ran away and hid). The dog must never think that it can hide from you. When the dog hides, it thinks it is safe. Caution must be used when getting the dog out of there. Don't stick ur hand in there, or with a big dog u may no longer have a hand. (only time i have ever been biten by a dog was with a small dog, i was careless because the dog was small). Once the dog is out, onto the choke chain. 2x she did this, but now she knows when i yell come, she better come running.
IT IS VERY IMPORTANT THAT THE DOG GETS IT IN HIS HEAD THAT YOU CAN SEE HIM, AND KNOW WHEN HE IS DOING BAD THINGS AT ALL TIMES.
So lets say u have trained him not to go into garbage. Then go inside. Close the door so i think u can not see him, but peak out of window or something.
If he starts rummaging thru garbage, run outside and yell NO. A few times of this he then things u can see him at all times

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Once the dogs understand NO. Reward them with treats, affection etc when they do what they are supposed to do. Dogs are pack animals, with a large dog it is important to establish that you are pack leader. When my wife drops something and screams one of the dogs always comes running to check out everything is ok. Sometimes my son when i am not paying attention climbs onto 1 of them, and then like bangs thier head like a bongo drum

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Excessive hitting etc is not nessary and bad. Also with a large dog, ussually his hide is so thick he can not really feel it, that is why u need choke chain.
Anyway it is ussually the fuking treehugger who thinks the fuking dog is a "little person", who has this untrained dog that ussually takes off some kids hand or turns on them and kills them.
Dogs are pretty smart. My male dog knows that he is now allowed to come into the house thru the back door (the one that is ussually open). So he just stands there and sticks his head in. But if u leave the front door open u did not specify he can not go in thru that door, do he will come in the front door and go out the back door

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Also if my son falls down or something and starts screaming, he will come inside the house to come and check it out. (You still want the dog to have common sense, if u abuse the dog, excessive choking etc, u will ruin them).
From my experience it takes approx 1-2 months to train a dog properly. As long as dog is intelligent ages does not really matter. I have successfully trained a dog which was approx 5 years old, which had been picked up by the SPCA after running with coyotes for several months. Again my tree hugging ex said it was not possible to train such an animal because of its "background". She was really pissed when the dog would listen to me, but not her, even after i had not seen the dog for over 6 months.
Another problem is never never make an exception with the rules. If dog is not allowed onto a chair for example, then don't allow him to sometimes go onto the chair he will get confused (dogs do not have an intelect like people do).