Questions & Answers about Non si è filata la paglia, ma la lana.
Why do we use è instead of ha? Doesn't the verb "to spin" normally take avere?
Why does the past participle end in -a (filata)?
It is agreeing with la paglia (the straw), which is a feminine singular noun. When an impersonal si is used with an explicit object, the sentence basically becomes passive (often called the passive si or si passivante). The object acts as the grammatical subject, so the past participle must agree with its gender and number.
How would the sentence change if we were talking about spinning plural things, like clothes (i vestiti)?
Could we put la paglia before the verb instead?