To obtain American citizenship, the zenith in terms of US immigration status, the main eligibility requirement is that you have to be a permanent resident (green card holder)
You have to meet certain other requirements too. You have to be 18 years or older. You have to be a permanent resident for five years to qualify. If you are married to a US citizen, then it is three years of permanent residency. You have to be in marriage to a US citizen and should be living with that US citizen spouse for the past three years of your permanent residency for the criteria to be applicable.
Another requirement is that you should be residing in the US for a continuous period before filing the citizenship form. If you are not married to a US citizen, you should reside in the US for a continuous period of five years after being lawfully admitted to the US as a permanent resident. If you are married to a US citizen, you should reside in the US for a continuous period of three years after being lawfully admitted to the US as a permanent resident.
A long absence from the US will break the continuity of your residence in the US for naturalization purposes. It, however will not affect your chance to return to the US as a permanent resident. Though an absence from the US of less than six months will not break the continuity of residence in the US for citizenship purposes, absence for a period of six months or more may result otherwise. However, if the break is between six months and one year, it can be excused if a valid explanation can be provided for the absence. If the break happens to be for over one year, the continuity of residence can be preserved if necessary steps are taken prior to the expiration of a year abroad.
In addition, you should also meet physical presence requirements. You should be physically present in the US for a certain number of months to be eligible to file the citizenship form. Before applying for US citizenship, you should have resided in your current state for at least three months.
Another important factor is that you should not have broken any US immigration laws and should not have been ordered to leave the US. You are required to prove at least 5 years of good moral character. Apart from these, you have to take an English language test and a Civics test. You have to prove that you can read, write and speak basic English and that you also have basic knowledge of US history and government.
Some applicants can be exempted if they have a medically determinable physical or mental impairment, where it affects one’s ability to learn English and Civics. And lastly, applicants should be prepared to take an oath of allegiance to the US to finally become citizen of the United States.