"The baby is writing a letter with a ballpoint pen."

Translation: 아기가 볼펜으로 편지를 쓰고 있어요.

[journey816]

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?

[nleconte]

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.

[MBoyer213]

Nicolas is right. Either way works, grammatically speaking, but 볼펜으로 편지를 쓰고 있다 sounds more natural

[nleconte]

Now I'm left to wonder how you know my first name (not that I'm very secretive about my identity as evidenced by my profile picture and my username) :)