Learn Icelandic: Allt fólkið verslar hérna.

The word allur has to match the gender, number, and case of the noun it describes. Because fólkið (the people) is a neuter singular noun in the nominative case, allur becomes allt.

Why is the verb verslar in the singular form when the translation says people?

In Icelandic, the word fólk is grammatically a singular neuter noun. Even though it refers to a group of people, it always takes a singular verb. Think of it like the English word crowd: you say the crowd shops, not the crowd shop.

Why does fólk have the definite article -ið attached to it?

When you use a form of allur to refer to the entirety of a specific group, the noun that follows usually takes the definite article. This is why all the people is allt fólkið.

Is there a difference between hér and hérna?

Both words mean here, but hérna is very common in everyday spoken Icelandic and often translates more like right here. You will hear both used almost interchangeably.