A form's grammatical features are given by a (possibly empty) list of Item Ids. Their purpose is similar to the purpose of descriptions: they disambiguate the form, and specify in which context the form is applicable. A "description" of the form may be composed by constructing a (comma separated) list of (the labels of) the items selected as grammatical features.
The UI should allow to see all the items selected as grammatical features for a given form, and allow that list to be edited, by adding or removing entries.
Both the function and the representation of a form's grammatical features is similar to the function and representation a lexeme's class and language. For this reason, presentation and manipulation of these things should work in a similar way.
Example user stories:
* as a novice editor, I see a form linked as "//gingen//, 3rd person, singular, imperfect"; I follow the link, and want to change "3rd person" to "2nd person". I expect this to work like editing a Lexeme's lemma, or an Item's description, since it is shown/used in the same way.
* as an experienced Wikidata editor new to Lexemes, I see the grammatical features "3rd person", "singular", and "imperfect" represented as (what seems to be) statements. I try to find the corresponding property, and fail. I try to formulate a query that uses these statements, and find no way.
* as an editor, I want to change the Lemma's class, and to adapt all the Forms' features accordingly. I expect this to work in similar ways, since the relationship between the lexical category and the Lexeme is similar to the relationship between the grammatical feature and the Form.