Complex sentence: Olge knew that aphasia and agraphia often occurred together, but he confirmed that the impairment of two different types of language (i.e. spoken and written) can vary in type and/or severity.
Split into simpler sentences: Olge knew that aphasia and agraphia often occurred together. However, he confirmed that the impairment of two different types of language (i.e. spoken and written) can vary in type and/or severity.