Concept guide
Proposition: deciding which candidate sentences are propositions, and separating 'is a proposition' from 'is proved true'
In an introductory classical context, decide for each candidate whether it is a proposition, and for each proposition decide whether it is true, with reasons: s1 '7 is prime'; s2 '4 is prime'; s3 'Is 7 prime?'; s4 'Compute 3+4.'; s5 'x>0' (x an unassigned integer variable), together with closed versions of s5: substituting x=2 and x=−1, prefixing ∀x∈ℤ, and prefixing ∃x∈ℤ.