Hi
At 18 weeks absolutely no road walking at all, your pup only needs to play in your garden and will recieve all the excerise he needs. At 6 months you can begin walking but only 5 mins in the morning and 5 mins in the evening, gradually increase this to 30 mins two times a day by the time your pup is 12 months. Your pup can still have unlimited play in your garden.
Re house training, the best method is to feed your dog outside and wait for them to wee and poo before you allow them back into the house. Most dogs toilet with their food, and this becomes a habit, then it is easy for the dog to undertand when outside he can potty, so when you let him out in between meals he goes then too.
Sharon

I'm not a specialist in Newfoundlands, but most giant breed puppies need very little exercise as such - 5 minutes per month of age is a general rule of thumb. What is important is that the pup is exposed to as many experiences as possible as soon as possible, ideally before 16 weeks, but unfortunately yours is already older than that. Socialisation is more important than exercise, so if you can take her to the town centre and sit on a bench with her so that she can get used to traffic, crowds etc, it would be very beneficial in the long run.
I have a giant breed pup and I started by sitting in the front with him either on my lap or by the chair.( I live on a main road with no front garden!!) Yes you do get some funny looks but also people do tend to come and say hello also the noise from the traffic also helps with all the socialisation.Puppy classes are great too, it also wears them out mentally.
As far as walking goes yes you can raod walk them but only 5 mins, a run in the park is better, as long as they have good rest periods between exercise then you should be ok.
Do this until they are 6 months old then gradually increase the lenghts of the walks.
Please dont keep them confined to the house and garden, take them in the car, I used to take mine to the local statley home just to sit and watch the world go by and get a cup of tea, sitting outside the supermarket is also a good place, use your imagination, just get puppy out and about.
I hope this helps
Julie