formative
Pronunciation: /ˈfɔːmətɪv/
adjective
- serving to form something, especially having a profound influence on a person’s development: his formative years
- relating to a person’s development: a formative assessment
- Linguistics denoting or relating to any of the smallest meaningful units that are used to form words in a language, typically combining forms and inflections.

Origin:
late 15th century: from Old French formatif, -ive or medieval Latin formativus, from Latin formare 'to form' (see form)