To scan a line is to weigh every syllable: is it LONG or SHORT? Two rules decide. A syllable is long BY NATURE if its vowel is long or a diphthong; it is long BY POSITION if a short vowel is followed by two or more consonants. Everything else is short. From those weights the whole metre falls out. Slide a syllable’s traits and watch the verdict flip.
Syllable weight has two sufficient conditions for LONG (heavy): BY NATURE — the vowel is long or a diphthong; or BY POSITION — a (short) vowel is followed by two or more consonants (which may straddle a word boundary). A syllable long either way counts as 2 MORAE; a light syllable is 1 mora. These weights, not stress, drive quantitative metre. A fail-loud self-check throws unless a long vowel OR two following consonants yields heavy, and a bare short vowel yields light. ◆ the scansion rules are exact, node-verified.
The two rules are exact; edge cases (a mute + liquid cluster can scan EITHER way — correptio Attica) are real licenced exceptions a scanner must judge, noted not hidden.