"We write a poem."

Translation: हम एक कविता लिखते हैं।

[seekhana]

If "we" happened to be all women, could this also be: हम एक कविता लिखती हैं।?

[deactivated user]

Yes. It should be accepted because we don't know who "we" are. We could easily be all female.

[VintiMajethia]

In common or general the helping verb would come as रहे. It can also come when the gender of the subject is not informed.

[dxrsam]

Correct, though लिखते हैं is not incorrect either, and mostly considered gender-neutral today.

[richard547513]

This wasn't accepted but I think that it should have been.

[JerryCurry3]

Agreed. Reported.

[Ellen147987]

It doesn't mark it wrong (in red) but does say "you have a typo".

[VintiMajethia]

Just a little correction, you would have to write हम सब एक कविता लिख रही हैं. Here the word लिख is a masculine noun and would remain as it is not depending on the gender of the subject. The helping verb would modify as रही .

[4buRmP5H]

Is the "ek" compulsory?

[vj-]

No, not at all.