Then, conj.
1. Than. [Obs.] Spenser.
2. In that case; in consequence; as a consequence; therefore; for this reason. If all this be so, then man has a natural freedom. Locke. Now, then, be all thy weighty cares away. Dryden.
Syn. -- Therefore. Then, Therefore. Both these words are used in reasoning; but therefore takes the lead, while then is rather subordinate or incidental. Therefore states reasons and draws inferences in form; then, to a great extent, takes the point as proved, and passes on to the general conclusion. "Therefore being justified by faith, we have peace with God." Rom. v. 1. "So then faith cometh by hearing, and hearing by the word of God." Rom. x. 17.
1 other Webster entry under this spelling
Then, adv.
1. At that time (referring to a time specified, either past or future). And the Canaanite was then in the land. Gen. xii. 6. Now I know in part; but then shall I know even as also I am known. 1 Cor. xiii. 12.
2. Soon afterward, or immediately; next; afterward. First be reconciled to thy brother, and then come and offer thy gift. Matt. v. 24.
3. At another time; later; again. One while the master is not aware of what is done, and then in other cases it may fall out to be own act. L'Estrange. By then. (a) By that time. (b) By the time that. [Obs.] But that opinion, I trust, by then this following argument hath been well read, will be left for one of the mysteries of an indulgent Antichrist. Milton. Now and then. See under Now, adv. -- Till then, until that time; until the time mentioned. Milton.
Note: Then is often used elliptically, like an adjective, for then existing; as, the then administration.
Shades of meaning
Fernald has no entry of its own for then; he lists it under therefore.
Where he writes about it:
Therefore, signifying for that (or this) reason, is the most precise and formal word for expressing the direct conclusion of a chain of reasoning; then carries a similar but slighter sense of inference, which it gives incidentally rather than formally; as, "All men are mortal; Caesar is a man; therefore Caesar is mortal;" or, "The contract is awarded; then there is no more to be said." Consequently denotes a direct result, but more frequently of a practical than a theoretic kind; as, "Important matters demand my attention; consequently I shall not sail to-day." Consequently is rarely used in the formal conclusions of logic or mathematics, but marks rather the freer and looser style of rhetorical argument. — from therefore