Learn Spanish (Latin America): Nadie dejó esta huella en el piso.

Because nadie (nobody) comes before the verb dejó, you don't use no. If you wanted to put nadie after the verb instead, you would need the double negative rule you are learning: No dejó esta huella nadie.

What verb form is used with nadie?

Nadie acts as a singular subject, so it always uses the third-person singular verb form (the él/ella/usted form). That is why we use dejó (he/she/it left) here.

Why doesn't esta have an accent mark?

Here, esta means 'this' and describes the feminine noun huella. It does not get an accent mark. The word está with an accent on the 'a' is a completely different word—it is the 'he/she/it is' form of the verb estar.

Does huella only mean a footprint from a shoe?

It can be any kind of mark, trace, or print left behind. It can mean a footprint, an animal track, or even a fingerprint (huella digital).

I learned that piso means 'apartment'. Does it mean that here?