Learn Swedish: Beslutet att sluta röka är svårt. - The decision to stop smoking is difficult.

This is an example of noun complementation. Just like in English, certain nouns in Swedish can be followed directly by att (to) plus an infinitive verb to explain what the noun refers to. Here, att sluta (to stop) acts as a complement to beslutet (the decision), telling us exactly what the decision is about.

Why is there no att (to) between sluta and röka?

The verb sluta (to stop/quit) is one of several common Swedish verbs, along with börja (start), bruka (usually do), and försöka (try), that often connect directly to the next infinitive verb without using another att. Therefore, "stop smoking" simply becomes sluta röka.

Why is the adjective spelled svårt with a -t at the end?

Adjectives must agree in gender with the noun they describe. The noun here is ett beslut (a decision), which is an ett-word (neuter). Even though the noun is in the definite form (beslutet), the adjective describing it across the verb "to be" must take the neuter -t ending, making it svårt rather than svår.

Would it be correct to say Beslut att sluta röka är svårt without the -et on beslut?