Learn Russian: Его долг больше ста долларов, у него одни проблемы. - His debt is more than a hundred dollars, he has only problems.

In Russian, using the plural form одни before a noun is a common way to say "nothing but" or "only." It paints a picture that the person is surrounded entirely by one thing. So, у него одни проблемы means "he has nothing but problems" or "he has only problems."

Why does the sentence use ста instead of сто for one hundred?

When you use a comparative word like больше (more), you use the genitive case to express the word "than" (more than a hundred). The genitive form of the number сто is ста.

In the phrase ста долларов, why does долларов stay exactly the same as in сто долларов?

It's a happy coincidence! When a number phrase goes into an oblique case like the genitive, both the number and the noun take that case. The genitive of сто is ста, and the genitive plural of доллар is долларов. It looks exactly like the basic phrase сто долларов (where the noun is only genitive because of the number 100), but here both words are fully in the genitive case.

I usually see долг translated as "duty." Can it mean both "duty" and "debt"?

Yes, долг is a versatile word. It can refer to a moral obligation or duty, but in financial contexts like this sentence, it translates directly to a financial "debt."