Complex sentence: The London merchant misinterpreted the handwriting understanding it to be a trade-name taken from the name of the river Tweed which flows through the Scottish Borders textile areas, subsequently the goods were advertised as Tweed, and the name has remained so ever since.
Split into simpler sentences: The London merchant misinterpreted the handwriting, understanding it to be a trade-name taken from the river Tweed that flows through the Scottish Borders textile area. Subsequently the goods were advertised as Tweed, and the name has remained ever since.