En general, el adjetivo viene después del pronombre; ocasionalmente se coloca antes por razones efecto literario o musicalidad de la frase, pero no es lo usual. En el ejemplo que usted pone, la forma adecuada es no entiendo la tarea (o el deber o el trabajo) de español.

Translating the previous answer to English: (because indy wants to Learn Spanish Using *English*) "In general, the adjective comes after the noun; occasionally one uses it before the noun in order to have a literary or musical effect in the sentence, but this is not the usual. In your example, the most appropriate way is "No entiendo la tarea [or "el deber" or "el trabajo"] de español."

sorry, I made a mistake, I was trying to say "In a general way, the adjective can be before OR after the noun"

In a general way, the adjective can be before of after the noun. For example: "El tigre furioso" o "El furioso tigre" = "the furious tiger". In you case, I think is better to say * "No comprendo las tareas de español". Zaijian!