Q:

Why is “de” used in “il n'y a pas de bus direct”?

I would need help in order not to mix up de and le or un
(What article can I use to refer to a thing correctly?)
I came across this sentence 

Il n'y a pas de bus direct?

But I couldn't understand why le wasn't used (or for that matter un, if you don't want to refer to a particular bus). 
Y avoir doesn't seem to be in this list of verbs that are succeeded by «de» here http://french.about.com/od/grammar/a/preposition_de_3.htm

A:

There are some situations, and negative sentences are one of them, where de is used instead of des (which is in this context the plural of un, there may be several direct busses).

Il y a des bus directs.
Il n'y a pas de bus directs.

(Note the plural for directs in both sentences).

