I do think this answer is off the mark, since I see no reason to think that the author of the passage would have used the word conversant if they'd known about it. It seems to me rather that they clearly meant exactly what Janus thinks they meant. There was no conflation of ideas; merely a non-standard use of a word. Native speakers do this sometimes, especially if they're not professional writers (and sometimes even if they are, when there is no copy editor to correct them).

Having read the original post on workplace.se, it seems to me the author has a good command of English for a software engineer. In this case, it also seems to me that they happen to have inserted a neologism into their text, although perhaps I'm just behind the times and this meaning will soon be found in dictionaries. Even if such a verb were recognized, however, I'd still rather write, "at least be able to talk about it."

At the end of this answer, you write the sentence exactly the say I do, word for word, based on what you think the writer is saying. So we both agree on that point. I also believe that writing a passage in such a way that it communicates the meaning you want to communicate is essential to correct writing, and that addressing what we think the author is trying to say is more than a "tangential point."

As noted in some of the answers below, it is possible that "my grades suffered" has neither of the connotations you suggested.

Whatever the justification for this particular construction, the point is that Poirot is not a fluent speaker of English, is it not? He is a fictional character and this is the way the author has written him.

I suppose you might say that a watch that's astronomical can help you track the positions of the planets relative to the plane that's ecliptic ... if you wanted to be funny. I don't think anyone would use those words that way in earnest.

It's the price of the Rolex that's astronomical, not the watch itself.

@DBS Some rations (such as odds ratios) work that way but not all do. You can have a ratio X:Y where X measures a part of something and Y measures the whole thing. It happens in geometry all the time.

Yes, and we have a natural preference for the more specific word. Clearly there is some reason why "number" is overwhelmingly favored over "quantity" in the Ngrams search FumbleFingers did.

People are always countable. There might be reasons of style to choose "quantity of people", as Dickens did in Great Expectations (quoted under FumbleFingers's answer). In example #2 the number of people who might be witnesses could have been very small indeed (which is the point of the quotation, in fact). Most of the time, I agree, there is no good reason to choose "quantity" rather than "number."

It's simpler than that. Quantity can be either countable or uncountable.

@PaulTanenbaum Quantity can be used either for amount or for number.

@Fattie Actually, quantity can be used with countable nouns as well as uncountable nouns, which is why the Dickens quote is perfectly natural. I was responding mainly to the assumption that the authors of #3 were native speakers. In this case it's probably more significant that the source is in a medical journal; how good is their copy-editing? Even the NY Times is a little iffy nowadays.

@FumbleFingers The authors of example #3 are Swedish (or at least affiliated with a Swedish university) so my guess is that this is not an example of written text from a native speaker. The lesson is to check what sources you're using.

@GlennWillen That joke seems to be based on the fact that when a performer says, "If you enjoyed the show, I'm Bob," they almost certainly want you to understand the sentence as a simple conditional (the "canonical" case): if you liked the show, help me build my reputation. A joke that I think is more in line with the third usage is, "I have lemonade if you want some. And I have lemonade if you don't want some."

@Jay I have long wondered whether someone attempting to Google a nasty computer problem they're having would get more focused results by peppering the query with foul language, on the premise that other people with the same problem are likely to use foul language to describe it too. But I've never scientifically tested that hypothesis.

The sentence in the title sounds fine to me. "First grade" is a little more specific than "school", that's all.

Under what circumstances would you do this? Is this some sort of "everyone introduce themselves" exercise, or a one-to-one conversation? Why do you need that particular form of response? Why does the prompt have to be a single sentence?