"The lamp is expensive."
Translation:Η λάμπα είναι ακριβή.
Don't forget about the genders ;)
The word λάμπα is feminine in gender, so the correct translation would be "η λάμπα είναι ακριβή".
(Φωτιστικό popped up because you used the article το. If there is a translation with the article you chose, then Duolingo doesn't mark the article as incorrect, but the noun -in this case, λάμπα)