There are several factors that determine how easy or difficult it is to get a green card by marrying a US citizen. Some basic factors that determine the level of difficulty of this process include factors like whether you marry outside or inside of the United States of America, and the kind of immigration status you have at the time of the marriage. The common assumption that just because a person marries a US citizen, he/she automatically gets the US citizenship is wrong. There is a certain legal procedure you have to follow.