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The website is incredibly customizeable, with the possibility to hone in on very specific chords and styles of dictations, not only will this help with dictation, but also with inversion and triads vs. 7th chords.
This is a good site for a definition and description of intervals. This would be good to read first to get an understanding of intervals, then be able to go and practice ear training to hear what you just read.
This website has good, simple definitions of form as well as listening examples with the music visible underneath each of the excerpts. Plus more links that lead to more information on specific forms.