発達 and 発展: language, towns, ports, and research development
Compare development in language or a town with a port or research, preserving overlap in urban contexts and the action of collecting materials.
Language and town development with 発達
発達 (はったつ) is used for development of functions, abilities, or an area. The example 「町は鉄道の開通とともに発達した。」 is translated as “The town developed with the opening of the railroad.” The town develops alongside the opening of the railroad. 発達 in this example is not restricted to language ability or a single function.
The example 「子どもの言葉の発達には個人差がある。」 is translated as “Children's language development varies from person to person.” Children's language development varies between individuals. This does not judge a particular child's level or set the same developmental timing for everyone.
Port and research development with 発展
発展 (はってん) refers to development of activities or society. The example 「小さな港が交易の町へ発展した。」 is translated as “A small port developed into a trading city.” The small port develops into a trading city. Along with the town example, this shows that both expressions can occur in urban development contexts.
The example 「研究の発展に役立つ資料を集めた。」 is translated as “I collected materials that would help advance the research.” Materials useful to research development are collected. Collecting them does not report that the research's development is already complete.
Urban overlap and the action actually completed
発達 occurs with language development as well as the town's development. 発展 occurs with a port developing into a trading city and with research development. Compare these representative emphases while preserving overlap in urban contexts.
Read individual differences in language development separately from collecting research materials. Target size or an internal-versus-external boundary does not fix the words into exclusive categories; follow the actual target and completed action.
Do the town's 発達 alongside the railroad and the port's 発展 into a trading city belong to exclusive categories?
Show the answer and why
No. Both expressions can concern urban development. Read the town's development and the port's change into a trading city, while keeping language variation and research material collection within their own sentences. Collected materials do not establish completed research development.