[David L.]

so I thought it would be 'the'.
it is so hard to decide what article to put in the sentence.
'The' is gramatically wrong?

Look at this:
"Dogs tend to wander all over the place. A dog wandered into my back yard yesterday and started digging up my lawn, looking for bones. I shouted at it trying to scare it away but the dog wouldn't leave.
"dogs` is general, I'm talking about 'dogs'.
"A dog" = first mention of a particular dog among all the dogs in the world.
'but the dog' = second mention and now referring to that particular dog, the one that wandered into my yard.

See how it happens in this conversation also:
Customer (ringing): Are there (any) tickets still available for HP3 tonight please? -no 'a', no' the' before tickets, because talking general about 'tickets for the movie'

Box Office: There are a few still available.
"a" is used because this is the first mention of more specific tickets - tickets that are available as opposed to tickets already sold.

He: Would you be able to hold them for me? I could pick up and pay for the tickets when we get to the cinema tonight.
'the' because now referring again to the tickets mentioned earlier, the ones referred to then as "a few still available" and which are now the tickets specifically to be put aside for him.

She: Sir. You have to understand that tickets for this movie are in great demand. The few tickets we do still have will probably sell in the next 15 minutes. We would require pre-payment if we were to hold them for you. You could pay by credit card over the phone.
'understand that tickets' :No use of article, because she is referring to tickets for this movie in general, tickets for that night, next week, and her experience of how tickets have sold out since the movie opened.
'the few tickets' : 'the' because we are continuing to talk about specific tickets, the few available that he wants her to hold for him.