The basic distinction is singular/plural. If you're talking about one quantity, you use LEVEL; if you're talking about more than one quantity, use LEVELS. "Inflation fell to its lowest level in 30 years" - singular; INFLATION (in a single country over a single period of time) is a single number; for example, "3% per annum".
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.