Noctem
Explorer
Spot on!
We don't have to know what the magic words for each spell actually are, we just know that there are some for each spell with a verbal component, and that it must be provided. If it is not provided, the spell is not successfully cast.
Let's say that the verbal component for, say, lighting bolt, the magic words which must be provided, is the phrase "So long as men shall breathe, or eyes shall see!"
If you just say "So long as men..." and then stop, have you provided the required verbal component? No. Does a partial bolt of lightning, roughly a third of the usual length, fry some of the baddies, with the bolt growing longer as you continue the phrase? No.
How do we know? Because if you just said "So long as men..." and the spell effect, or part of it, appeared, and then you refused to complete the phrase, then the spell effect appeared even though you never provided the verbal component! You can't just go around shouting "So...!" and expecting your spells to work anyway! You've failed to provide the verbal component (the whole phrase) and so, RAW, the spell fails.
This is why we know that the components must be fully completed before the spell effect begins, because until you complete the components, you haven't provided the components.
When the Ready action lets you ready a spell, then the wording that lets you fully 'cast' the spell as an action (the components/spell slot) and later use a reaction to release the spell effect, relies on the already existing spellcasting rules that mean the casting process must complete before the spell effect begins.
That's exactly what I'm saying too Arial. 100% agreement there.