Marks the one course left once the alternatives are shut off
框 · Frame
[circumstance shuts off options], [subj] 只好 [the one remaining action].
觸 · Trigger
Every preferred option is blocked and you are settling for the single course that remains.
序 · The move
1State the situation that closes off the alternatives, often a fact already set (rain, a closed door, a missed train).Are the better options genuinely blocked, not merely unchosen?
2Place 只好 before the one action that remains.Is what follows 只好 a verb phrase, the action taken — not a noun or a reason?
3Read it back: the action is taken because nothing else is open.If the speaker actually preferred this action, 只好 is wrong — it carries settling, not choosing.
例 · Examples
1外面下起了started (raining)大雨heavy rain，我們只好have no choice but to留在stay in飯店hotel裡。
It started pouring outside, so we had no choice but to stay in the hotel.
只好 settles for the one course left after others are blocked by circumstance; 只能 states a fixed limit on ability or permission (this is all that is possible at all). 沒帶傘，只好淋雨回家 settles; 這台機器只能印黑白 states the limit.
如果…就… lays out a hypothetical and its result; 只好 reports a real situation that already forced the hand. No 如果 sets up 只好.
外面下大雨，我們只好飯店。 ✗ → 外面下大雨，我們只好留在飯店裡。 ✓ (只好 takes a verb phrase, the action taken, not a bare noun.)
我喜歡這家店，只好常來。 ✗ → 我喜歡這家店，所以常來。 ✓ (只好 is settling under constraint; a preferred action takes 所以, not 只好.)
English have to / can only also covers plain obligation and ability; 只好 is narrower — the one course remaining after the better ones are shut off. For a fixed limit reach for 只能, for an order or rule reach for 必須.