The difference between obligatory and non-obligatory grammatical categories in 'thinking for speaking'

Slobin (1996; 2003; 2008) has shown that the presence of a grammatically encoded category directs speakers in the ‘thinking for speaking’ process. In experiments with children, Slobin focuses on the expression of progressive aspect, e.g. the present and past continuous in English as compared to other languages which lack such a category. However, Slobin does not distinguish between obligatory and non-obligatory categories: while both are encoded form-meaning pairings in a language’s grammar (cf. Levinson 2000, Belligh & Willems 2021), only the former must be used in speech in specific contexts. We explore whether the influence of an encoded category with regard to ‘thinking for speaking’ depends on being obligatory or non-obligatory.