There was an interesting question recently on the TESL-L email list. One person mentioned that the presentation of grammar point: "be able to do, was able to do, etc." in a certain textbook was confusing because it followed a chapter in which "could do" was presented. "Wasn't able to do" sounded unnatural to me (compared to "couldn't do" which sounded OK) so I did a google search to see if people use this term on the web and it turns out they do.
"Wasn't able to install" seemed to be a big one. That got me thinking of collocations as I recently taught my students how to use the wonderful Oxford Collocations, using obtain as an example. "be able to" collocates with obtain. I think it would sound unnatural to use "couldn't obtain" now that I think about it.
Perhaps teachers can turn this confusing presentation of grammar forms into a positive by pointing out that natural sounding English depends on collocations and having students keep journals (for example of which actions go with "could" and which go with "be able to"). Of course, keep in mind that the book may be wrong and when your book uses unnatural collocations students must be made aware of this.
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