Learn Hardcore Norwegian: Hjelm kreves på sykkelstien. - A helmet is required on the bike path.

Sykkelsti: a bike path, usually separated from car traffic (often shared-use or alongside roads/parks).
Sykkelfelt: a bike lane painted on the roadway.
Sykkelvei/sykkelveg: a dedicated cycle road/way. The sentence specifically refers to a path: sykkelstien (the bike path).

Kreves is the present passive of kreve (to require). The -s ending is the Norwegian s-passive, which often corresponds to English “is/are + past participle.” So Hjelm kreves literally means “Helmet is required,” with the doer (e.g., rules/authorities) left implicit.

Why is there no article before hjelm? Why not en hjelm?

Norwegian often uses the bare singular to make generic statements about what is required/allowed/forbidden. Hjelm here means “helmets (in general).” Adding en would sound like “a helmet is required” (as if exactly one unspecified helmet), which is not how such rules are phrased. Use the bare singular for rules and general requirements:

Bilbelte er påbudt. (Seatbelts are mandatory.)

Why not Hjelmen kreves with the definite form?

Hjelmen refers to one specific, identifiable helmet. The sentence is not about a particular helmet; it states a general rule. Hence the generic bare singular hjelm is correct.

Could I use the plural and say Hjelmer kreves?

Grammatically possible but stylistically odd. For category-level rules, Norwegian strongly prefers the bare singular: Hjelm kreves or Hjelm er påbudt. Plural would suggest multiple helmets per person or sound less idiomatic in a rule statement.