Why is the Imperfect considered easier than the Preterite?

Does anyone know what my spanish teacher means by this? i need to use plural forms of the verb ser. and she asked me to use a couple of adjectives to describe the boys collectively and then the girls collectively?

Las calles estén sucias. es recomendable que es una vergüenza que es preferible que es preciso que

Ineed with spanish homework. translation of task: 11. fill out with the correct form of estar or hay 12. fill in fill with the verbs hay, estar and ser.