A caption line is not always a sentence
Automatic caption systems optimize for timing and recognition, not for grammatical completeness. A line may begin in the middle of an idea or end before the speaker finishes it.
Context resolves small words with large consequences
Pronouns, implied subjects, technical abbreviations, humor, and forms of address can change meaning depending on who is speaking and what came before.
Subtitle quality is also editing
Good translation is only part of the job. Readability depends on punctuation, sentence grouping, speaker turns, cue length, and how the text arrives against the voice.
That is the problem Mono is trying to solve
Mono uses the caption timeline as structure, wider context for meaning, and a Final pass for readable cues so the result feels closer to edited subtitles than a stream of isolated translated fragments.