Learn French: Il n'y a aucun sentier ici. - There is not a single trail here.

The word aucun (not a single / no) replaces pas. In French, you generally do not use pas alongside another negative word. The structure here is ne ... aucun, which works just like ne ... jamais (never) or ne ... rien (nothing).

What is the difference between saying Il n'y a pas de sentier and Il n'y a aucun sentier?

Both refer to an absence of trails, but aucun is stronger. While pas de just means "no" or "not any", aucun specifically emphasizes that there is not a single trail here.

Why is sentier singular here? Doesn't English sometimes use the plural ("There are no trails here")?

Even though we often say "no trails" in English, aucun literally translates to "not a single". Because "a single" is inherently singular, the noun that follows it (sentier) must also remain singular in French.

How does the placement of ne... aucun work with the expression il y a?