Learn Dutch: Wij waren de nieuwe camera aan het testen.

To say "were testing", we use the past tense of the verb zijn (which is waren for "we") followed by aan het and the full verb testen. It literally translates to "were at the testing".

Why does de nieuwe camera go in the middle of the sentence?

In Dutch, the continuous part (aan het testen) gets pushed to the very end of the clause. The object (de nieuwe camera) must come between the conjugated verb (waren) and the aan het block. You cannot say "Wij waren aan het testen de nieuwe camera" like you would in English.

Why is it nieuwe instead of just nieuw?

Because camera is a de word (de camera). When an adjective comes directly before a noun that uses de, you add an -e to the end of the adjective.

Could I use we instead of wij here?

Yes. We is the normal, unstressed word for "we". Using wij puts special emphasis on the people doing the action: we were testing it, perhaps as opposed to someone else.