If they are here legally, then yes they must pay income tax. This is assuming that they are working at a place that keeps track of employees salary. If they are just visiting, they obviously won't have to pay anything.
A Federal income tax refund is not taxable income (for state or Federal purposes) in the year a taxpayer receives it. A state income tax refund for a previous tax year, however, may be another story....
You are the only one that has all of the necessary information that will have to be reported on your 1040 FEDERAL income tax return for the year in order to do the calculation for the numbers that...
On a standar IRA, Yes (you didn't pay tax on the $ contributed or as it grew). On a Roth IRA, (where you paid the tax on the income before contribution), No.
I assume that this question is about an income tax refund, and not about an income tax return (which is the form you file with income tax authorities every year, along with any income taxes you still...
Why use highways, bridges and airports, accept the protection of the Armed Forces, go to schools that get Federal money, or any of the other thousands of things you take for granted that are paid for...