why is an article "a", but not "the"?

[Huong]

English Grammar Tests, Elementary Level

ESL/EFL Test #98 "Articles in English Grammar", question 6

Do you want to watch ......... movie with me tonight?

(a) a
(b) an
(c) the

English Grammar Tests, Elementary Level

ESL/EFL Test #98 "Articles in English Grammar", answer 6

Do you want to watch a movie with me tonight?

Correct answer: (a) a

Your answer was: incorrect
Do you want to watch the movie with me tonight?
_________________________

why"a" but"the"?

Huong

[Mister Micawber]

.
Well, actually both are OK, Huong, but we are expected to read the question as a first mention of the movie. I will ask the question writer about changing it, thank you.
.
_________________
Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mr. Micawber