Questions & Answers about Zij strijkt vaak een jurk.
Why does een jurk come after the adverb vaak?
Because een jurk is an indefinite direct object (it uses een instead of de or het), it is placed after adverbs of time, manner, and place. Since vaak (often) is a time/frequency adverb, the indefinite object must follow it.
How would the word order change if she were ironing the dress instead of a dress?
If the object were definite (de jurk), it would move in front of the adverb. The sentence would become: Zij strijkt de jurk vaak. Definite objects generally come before time, manner, and place adverbs.
Could I start this sentence with the word Vaak?
Yes, but remember that doing so triggers inversion. If you put Vaak at the beginning of the sentence to emphasize how often it happens, the verb must stay in the second position, pushing the subject after it: Vaak strijkt zij een jurk.
Is there a reason to use Zij instead of Ze here?
Grammatically, both are correct. Zij is the stressed form, which you use when you want to emphasize that she is the one doing the ironing, rather than someone else. Ze is the unstressed, neutral form and is more common in everyday speech unless that emphasis is needed.