Assuming the resident keeps the relevant cards straight, he or she can meet the requirements of both counties and ultimately qualify for citizenship in Canada after three years and citizenship in the United States after five years. Once citizenship is acquired in either country the necessity of residence no longer applies. You can be a citizen of the United States and live in Canada for the rest of your life without losing your citizenship. The opposite is also true.
Even in the Canadian case, the resident would be wise to enter the country on the maple leaf card. It is important not to confuse these two. And it is important to show only one relevant card when entering the relevant country in order to stay away from further questioning.
When entering the United States a permanent resident must present his or her green card. This establishes the prima facie right to enter the country and, as long as you have not been away from the United States for more than six months, you will be routinely allowed to return. The key is not to be absent for more than six months at a time since that is the working formula employed by U.S. Customs and Border Protection officers to determine whether you should be questioned by them about your absence.
If you are lucky enough to have a green card proving permanent residence in the United States and a maple leaf card (same as a green card in the U.S.A.) proving permanent residence in Canada, your goal should eventually be to become a citizen of one or both countries. In the meantime, however, you can honestly abide by the requirements of both countries if you maintain domicile in the United States but also ensure you are in Canada sufficiently over the course of each five years to maintain your Canadian status as well.
In both instances, U.S. and Canada, there are exceptions for residents working abroad due to military service, or employment with a firm connected to the home country for example. Such instances merit a separate discussion beyond what is covered here. However, most people are impacted by the general rules described above.
Under Canadian immigration law, permanent residence is defined by an objective physical presence test. According to Canadian law, as long as you are physically present in Canada for a cumulative period of not less than two years (or 730 days) in any five year period, you are maintaining permanent residence in Canada. So, for example, in any five year period, A Canadian resident could choose to live three years straight in the United States provided that he or she then returned to live in Canada for the last two years.
Under U.S. immigration law, permanent residence is defined by domicile. Domicile is a legal term that might best be described as where your "home" is as opposed to where your "houses" might be. For example, a person might have a residence in Paris, London and New York, but only one domicile - that is, a place he or she regards as "home". Domicile, under American law, is an ingredient that connotes long term attachment to a place of residence.
The quick answer to this question is yes.
The concept of permanent residence is different in the United States than it is in Canada. The difference in definition of the term makes it possible, albeit not easy, to maintain permanent residence in the two countries. The issue has become particularly important ever since both countries adopted the approach of regular reviews of permanent resident status: every five years in Canada, every ten years in the United States.
Yes. If the immigration officer at the port of entry learns of a romantic tie between you and someone in the United States they are likely to ask a lot of questions and possibly bar your entry. That is because such a tie strongly suggests good reason to believe you will not likely leave the US at the end of your period of authorized stay. Be prepared to answer a lot of questions if the reason you are going to the US has anything to do with romance.
You should have a driver's license, a social insurance card in Canada or something similar from abroad, a health care card, some proof of a job, a bank statement, membership cards in various clubs or associations and other such documents. Hopefully your address on all documents is the same. Even pictures of your family help.