When listing keywords for a paper written in English, is it better to write keywords in singular or plural form?
I am interested in both keywords stored in meta-data:
and keywords listed at the beginning of the paper:
Honestly, I think that keywords are now de facto irrelevant. The issue is this: when was the last time that you actually searched for a paper by keyword? In practice, literature discovery is now more typically done by means of modern search engines, which will generally disregard such minor distinctions as singular vs. plurals in any case.
I would recommend checking the thesaurus of a database that is likely to be searched for your discipline for relevant keywords and compare yours to the keywords indexed within that database to match your terms to an appropriate taxonomy. Singular or plural form is unlikely to matter, but listing phrases instead of breaking the keywords down to strict terms is unlikely to be useful to anyone at the publishing companies or managing your institutional repository that is converting the terms to a controlled vocabulary. Speak to a reference librarian for assistance locating a database thesaurus. Also, to the point above about the terms not mattering although this is true in disciplines with precise taxonomies which are mapped only to that discipline (as is often the case in computer science or engineering) many disciplines still use terminology that is not strictly mapped to their disciplines. In these instances controlled vocabularies (within the databases) remain relevant to resolve the ambiguity of terms from unrelated disciplines when searching.