Considering https://dictionary.cambridge.org/grammar/british-grammar/could (point "Could + smell, taste, think, believe, etc."), I assume that one is allowed to use "could" to refer to single achievements in the past when the sentence is in the passive voice.
For instance:
The probability of obtaining suboptimal solutions could be lowered by simply making the optimiser search more but similar waveforms
is better than:
The probability of obtaining suboptimal solutions was able to be lowered by simply making the optimiser search more but similar waveforms
Two points:
- obviously, only the context helps the reader to know that "could be" does not mean "is likely to be/may be". In the sentences before this one, I used exclusively simple past, so the tense should be clear in my humble opinion.
- the latter sentence also causes a problem since "able" is used for a thing, which is usually reserved for living beings (at least in standard English as far as I am aware)
For me, "be" is also a stative verb like "smell, taste, think, believe" mentioned in the Cambridge article. I know that I can rewrite the sentence and use phrases like "it was possible" or "one managed to", but I would like to avoid these here.
So am I right in thinking that one can use "could be" in the passive voice to mean "was able to be", which sounds awkward?
PS: I am already aware of this article here: "was able to" vs "could", but it didn't help me in this regard. Help would be much appreciated!