Two Patterns, Told Apart by What Precedes
1. ますstem + 次第 — as soon as.
- 到着し**次第**ご連絡します。(I'll contact you as soon as I arrive.)
2. Noun + 次第 — depending on.
- 天気**次第**です。(It depends on the weather.)
A verb stem gives sense 1; a noun gives sense 2.
Sense 1 Cannot Be Past
As-soon-as 次第 always looks forward, so the second clause must be future — a plan, a promise or a request:
| ○ 終わり次第帰ります | I'll go home as soon as it's done |
| × 終わり次第帰りました | past — use たらすぐ |
Against たとたん and てから
| Pattern | Timing |
|---|---|
| たとたん | the instant it happened — past, surprising |
| てから | after — any time gap |
| 次第 | the moment it becomes possible — future, planned |
次第 is the business register of "as soon as", and appears constantly in email.
次第で and 次第だ
次第で adds "depending on the circumstances": 結果次第で方針を変えます. And あなた次第です means "it's up to you".