Take another look at the sentence:
Maria knowed the futility of her task that October afternoon. As she swept off the front sidewalk, more leaves fell from the trees overhead, sprinkling the concrete with color.
The first verb, knowed, is the problem. The sentence requires the simple past. Knew is the correctly spelled form.
|Verb||Simple Present||Simple Past||Past Participle|
Fell, the verb you chose, is perfectly correct. Before you continue, review the rules for irregular verbs.