Complex sentence: Establishing the forms of the disease ("Classification") can be an important first section, but if the classification depends heavily on understanding the etiology, pathogenesis or symptoms, then that section should be placed later in the article.
Split into simpler sentences: Establishing the forms of the disease (Classification) can be an important first section. However, if such classification depends heavily on understanding the etiology, pathogenesis or symptoms, then that section may be better moved to later in the article.