Marks an outcome as unavoidable given the situation
框 · Frame
[situation], 不免 [verb / state that follows]
觸 · Trigger
You want to say the outcome arrives on its own — given the setup, there is no slipping past it.
序 · The move
1Set the situation that makes the outcome unavoidable in the front clause.Does the outcome follow from the setup on its own, with no one choosing it?
2Place 不免 right before the reaction or state that comes anyway.Is what follows a natural consequence (a feeling, a reaction, an encounter), not an action someone is forced to perform?
3Read it as 'cannot avoid' — the escape is blocked.Could you swap in 難免 and keep the meaning? If yes, 不免 fits; if the sentence needs a person compelled to act, use 不得不 instead.
不得不 forces an agent to act — they have no choice but to do X (不得不轉學 = has no choice but to transfer schools). 不免 makes an outcome arrive on its own — no one acts, the result just comes (不免擔心 = can't help worrying). 不得不 = compelled to do; 不免 = bound to happen.
難免 and 不免 both mark something as hard to avoid and are often interchangeable. 難免 says it is hard to escape (some still might); 不免 says it is not escaped (it comes). 不免 reads a shade more certain and more written.
✗ 我不免去學校。 → ✓ 我不得不去學校。 (a person forced to take an action needs 不得不; 不免 marks a consequence that arrives by itself)
✗ 你不免要小心嗎？ → ✓ 你難免會擔心。 (不免 states an unavoidable outcome flatly; it does not head a question or a command)
✗ 聽到這個消息，他不免了哭。 → ✓ 聽到這個消息，他不免哭了。 (不免 sits directly before the verb of the reaction; do not split it from what follows)
English reaches for 'have to' or 'must' for both compulsion and inevitability; 不免 covers only the second — the result that comes anyway — and never the person who is made to act.