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When your sentence is about one specific success
Single success, one occasion: if you're reporting one finished result, use was/were able to or managed to, not could. Only a few students were able to pass the exam, or they managed to pass it; could pass reads as general past ability, so it doesn't say anyone actually passed.
General ability, repeated or ongoing: when the sentence covers a skill or something that happened again and again, both forms work. When I lived by the coast, I could swim every morning or I was able to swim every morning; either reports the same habit.
One event with real effort: both forms are possible, but managed to foregrounds the effort. We were running late, but we were able to catch the train; with something harder, say she managed to reach the shelf by standing on a chair.
Perception verbs, one moment: here's the main exception. With see, hear, smell or taste, could works even for a single occasion: From the balcony, I could hear the concert clearly sounds right, and I was able to hear it would sound stiff. Your test: ask whether the verb reports a perception rather than an achievement.
One case where could still works for a single moment
Could describes an ability someone had over time, not a completed result on one occasion. So when your sentence is about one specific past event that actually happened, could doesn't fit, and you need was/were able to or managed to instead. Yesterday I managed to fix the car in an hour reports one finished success, while I could fix cars describes a skill I had for years. Before you settle on the wording, check what the verb reports: a perception or an achievement, because that decides which case applies.
Perception: you can usually keep could even for one occasion. From the balcony, I could hear the concert clearly sounds natural, while I was able to hear the concert would sound stiff there.
Achievement: the exception doesn't apply, so use was/were able to or managed to for the finished result. I was able to find my seat before the lights went down works; could find wouldn't say you actually did.
The verbs in this exception are the senses: see, hear, smell and taste are the common ones. It's a common pattern, but not absolute, so treat it as a tendency rather than a rule.
Your quick check: ask whether your sentence reports something your senses registered, or something you accomplished. If it's a perception, could is usually fine; if it's a success, reach for was able to or managed to.