Interesting, because often the ways that native English speakers would use these phrases is actually incorrect.

1.  She is only in the United States for a year = technically means that she will not be anywhere but the US for a year, but is actually used to say that she will only spend a year in the United States, no longer

2. She has been only in the United States for a year = she has not been anywhere but the United States for a year

3. She is in the United States only for a year = means she will only spend a year in the US, no longer.

4.  She has only been in the United States for a year = technically means the same as 2), but is actuallly used to say that she came to the US a year ago.

5.  She has been in the US for only a year = she came to the US a year ago.