Linked from
The 75 pages that link to First-order logic, each with the reason it gives.
Compactness theoremNarrower topic: The theorem applies to sets of sentences in this logic.
Hoare logicNarrower topic: Its predicates and quantifiers often express program assertions.
Diagonal lemmaNarrower topic: The lemma is stated for formulas and sentences in a formal language.
Prenex normal formNarrower topic: Prenex form is a normal form for formulas in this logic.
Skolem normal formNarrower topic: Skolem normal form is defined for formulas in this logic.