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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:

  1. 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.
  2. 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!

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  • This is a truly awful sentence -- but not for the reasons you mention. Beyond all that, I can't tell what it means between two possibilities. Are you (1) speaking theoretically like "if only we'd thought of it, it could have been improved by...."? Or is this (2) something you actually did do, and if so, did it work? If #2, that should just read: "We got better answers by having the optimizer also look for similar wave forms." Ditch the big words and tortured verb forms. Be direct.
    – tchrist ♦
    Feb 21 '20 at 19:25
  • Thank you tchrist! Ok, sorry about the complicated sentences. Apart from the meaning of the sentences, is it ok to use "could be" in lieu of "was able to be" whenever it occurs?
    – ExOrbitant
    Feb 21 '20 at 19:50
  • 'It was possible to lower ....'
    – Edwin Ashworth
    Feb 21 '20 at 20:03
  • Thank you Edwin! Yes, this is also a way, but would "could be" wrong? I would like to enrich my writing. I used "it was possible", "one managed to", ... already quite often.
    – ExOrbitant
    Feb 21 '20 at 20:06
  • You say "obviously, only the context helps the reader to know that "could be" does not mean "is likely to be/may be". This is not true. In your statement "could" appears to have the meaning of "can possibly be" - despite having a previous "probability" in the sentence. You need to use "can" as what you are expressing is a universal truth, as the reader is, and always will be able to lower the probability of obtaining suboptimal solutions by the method described.
    – Greybeard
    Feb 23 '20 at 9:16
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You are referencing the wrong sense of could (single achievements in the past) in your dictionary.

Your sentence uses could in its sense of past (or reported) ability—in the same way that we use can for present ability.

The voice—active or passive—has no bearing here.

Using your dictionary, here is the relevant entry for could:

past simple of "can", used to talk about what someone or something was able or allowed to do
Source: Cambridge Dictionary—could (CAN)

Let's look at some simplified sentences for illustration . . .

Present ability (active): Researchers can lower that probability by making the optimiser search more waveforms.

Present ability (passive): That probability can be lowered [by researchers] by making the optimiser search more waveforms.

Researchers currently have that ability. Are you with me so far? Next . . .

Past ability (active): [In the past] researchers could lower that probability by making the optimiser search more waveforms.

Past ability (passive): [In the past] that probability could be lowered [by researchers] by making the optimiser search more waveforms.

Researchers had that ability in the past. Following?

We also "backshift" can to could when reporting, even if the ability still stands; researchers still have this ability even though we're using the past tense:

Reported ability (active): [They observed that] researchers could lower that probability by making the optimiser search more waveforms.

Reported ability (passive): [They observed that] that probability could be lowered [by researchers] by making the optimiser search more waveforms.

If you apply context to your original sentence, it likely matches the example immediately above—reported ability in the passive voice.

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  • Thank you! This was helpful! I guess my sentence is correct then, right? It‘s interesting that you interpreted my sentence as an ability case, which definitely also makes sense. I thought about it as a single achievement in the past as every study can have different outcomes and one never knows what this time will be achieved. Incidentally, it’s funny that we use „able to“ (able comes from ability) single achievements. If I were to have the single-achievement sentence in the passive voice, could I use „could be“ to avoid „was able to be“?
    – ExOrbitant
    Feb 23 '20 at 7:33
  • When can we use „able“ with things? Results were able to be lowered (sounds awkward); one may say that „a computer was able to run through these calculations fast“ or „a train is able to run at high speeds“, but I think natives prefer for some unknown and non-intuitive reason „capable“ for things. Could you shed some light on this related topic as well, please?
    – ExOrbitant
    Feb 23 '20 at 7:40
  • @ExOrbitant: Your first sentence is correct for past ability. If you do mean past specific achievement, then you can say in the active voice: [Last Tuesday] researchers were able to lower that probability by making the optimiser search more waveforms. In the passive voice, you might hear this: [Last Tuesday] that probability was able to be lowered by making the optimiser search more waveforms. But it doesn't make sense, because the agency has changed; now the probability has the ability, not the researchers. You can't substitute could because that flips the sense back to past ability.
    – Tinfoil Hat
    Feb 23 '20 at 16:29
  • If you must use the passive voice here, skip the periphrastic modal: [Last Tuesday] that probability was lowered by making the optimiser search more waveforms. It's generally fine to use able for things or people. But see what Fowler has to say about this whole matter.
    – Tinfoil Hat
    Feb 23 '20 at 16:30
  • Thank you @Tinfoil Hat! According to Fowler's book (which helped me a lot for my understanding of 'able to'), it is okay to you "could" to replace "able to be" in the passive voice (see the last sentence). I am still not convinced because why can one say 'I could hear/believe/understand/... that' (cf. Cambridge, single occurence in the past) but apparently not "could be lowered" (single occurence in the past). I really don't want to be annoying, I would just like to understand it.
    – ExOrbitant
    Feb 23 '20 at 19:30
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No, you are not right in thinking that one can use "could be" in the passive voice to mean "was able to be", because "[...] when we talk about one special occasion in the past, we use be able to (positive) and couldn't (negative)."

Source: englishclub.com

This rule is confirmed by esl-lounge.com:

Specific Ability

If we want to talk about someone's ability to do something at a specific time in the past, we must use "to be able to".

I studied a lot for this exam and I was able to finish it easily.

Here we cannot use "could". This is not a general ability

Which is applicable to your example, because you are, I assume, recounting the steps that you have carried out in a study, which has been conducted at a specific point in time.

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  • The examples you provided are in active voice so they are not comparable with mine. Second, have a look at the Cambridge dictionary (mentioned above), there are situations where you use „could“ to refer to single past activities. Please provide links that come from sources of authority like Cambridge or Oxford.
    – ExOrbitant
    Feb 21 '20 at 20:52
  • "The police were not able to save him" is fine as a negative sentence. There are times when "could" does mean "able to", there are other times when it doesn't. So that rule doesn't always work.
    – CJ Dennis
    Feb 21 '20 at 22:41
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No, I don't think so.

Ability is not synonymous with what is possible.

"Could" is a conjugation of "can"--and when you say that you "can" do something you are not quite saying you are "able" to do something. There are connotative differences.

(Amusingly, in English the word "could" has additional connotative meaning beyond "can"--I know legally they are different versions of the same word, but they express different ideas. They feel different.)

No sources, I just like English.

Perhaps obvious thought: This is the kind of problem we bump into when we commit to the passive voice. There are lots of ways of writing this sentence which avoid this problem and result in a clearer sentence... but you may have to step back from passive.

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