"The baby is writing a letter with a ballpoint pen."
Translation:아기가 볼펜으로 편지를 쓰고 있어요.
Need help understanding why "by pen" (how) comes before "a letter" (what)? Isn't the how placed directly in front of the verb in a sentence?
Korean is rather flexible. As long as you're putting the verb at the end, you're often safe with any combination.
Yet, some are more common/natural than others.
Your options sounds good to me too.