"I want a new pen."

Translation: 新しいペンが欲しいです。

[darthoctopus]

新しいペが欲しいです

[Synargy]

新しいペンが欲しいです。

[vngdhuyen]

I haven't mastered は/が completely yet. can it be 「新しいペンは欲しいです」 here?

[KeithWong9]

Yes, if you want to stress the 新しいペン part.

[Scintilla72]

Still not accepted as of January 2021; I'll have to report it.

[kit_katze]

Why it used が instead of を?

[KagayakuSeiza]

Basically because ほしい is an adjective, as opposed to a verb. The を particle is used to indicate a direct object, typically followed by a verb. But Japanese typically doesn't use a verb in this case when expressing that you want an item, so you wouldn't use を because the item you want isn't acting as the direct object.

[KeithWong9]

ほしい is an adjective so it does not have an object so it is incorrect to use を.
が is used to represent the target of hope, preference and ability like ほしい, たい, 好き, きらい, できる

[TarantulaB1]

what's the difference between
あたらしい and あらたな？

新しい(あたらしい) ペンが ほしいです。
新たな(あらたな)ペンが欲しいです。

[KeithWong9]

I think 新（あら）た is used mainly for abstract nouns like 新たな時代（じだい） (new era) and 新たなペン seems a bit odd to me.

[toastedbunz]

Can I leave out the です?

[KagayakuSeiza]

In informal speech, yes.

[Robert229907]

It's a little frustrating that Duolingo wont accept it as an answer, however. Sometimes they accept informal speech and sometimes they don't. It should be more consistent.