Why did Itachi seek Tobi's help? In "Episode 359 : The Night of the Tragedy", it was revealed that Itachi sought the help of Tobi to massacre the Uchiha clan. So the following questions arise out of this situation:


Did he know Tobi's real identity?
Why did he take his help to eliminate the clan (irrespective of whether his identity was known or not)? 
Why did he help Tobi by not revealing that he was the village infiltrator as Itachi was always working for the Village?
Why did Tobi agree to help Itachi eliminate his own clan?


Ideally, each question here should be a separate question, but in this scenario all these are closely inter-related and thus have been posted as a single question.
The things he told Sasuke during their battle suggests that he thought that Tobi was Madara.
Either because eliminating the entire clan on his own was too tough a mission for him, or because he wanted to infiltrate the Akatsuki to gather information. Also, he had Tobi promise to not hurt the Hidden Leaf or Sasuke.
Because that would blow his cover. He was still protecting Sasuke and the Leaf and revealing his position would make him lose all the things he could have gotten due to #2.
Because, as Madara, Tobi too had a grudge against the Uchiha. Also, he could use someone like Itachi in Akatsuki.