Very important question, and I really didn't know the answer until I started reading. Main conclusions:
- There is some connection. If the plural form exists and is relevant, then it's imprtant most of the time.
- The position of your app in using Singular keywords will be different from using Plural, most times.
- An exact match of keyword really counts, so pic the form that will generate the highest amount of dowenloads and higher position in the search results.
- For English, using Singular is usually better and safer.
That's just a part of it. You can read it all here: