What are some main lines of the Horwitz Defense (d4 e6) besides the French? I'm not the most experienced player by a long shot and appear to be below average, but I know how to play the French Defense quite well and I tried e6 in response to d4 a few times because I don't really know many d4 openings and got a French out of it. How do I respond to c4 or any other common ones (maybe Nf3 or g3?)? I've looked at opening databases and know popular responses, but I want to know why these moves were made. If anyone knows of a video describing it well, that'd be preferable, but I'll gladly take any advice I can get.
Basically, you have a lot of options left to transpose into other openings. I remember having read a book calling this opening the Franco-Indian for this very reason. I do not expect that many White players will actually play 2. e4; they played 1. d4 for a reason and are usually not experts in the French opening.

After 2. c4, you can:


transpose to the Orthodox Queen's Gambit with 2... d5
transpose to the Dutch opening with 2... f5 (but you lose the ability to play the Leningrad system)
transpose to the Nimzo-/Queen's Indian with 2... Nf6
transpose to the Benoni with 2... c5 (thanks @user1583209)
2... Bb4+ is about the only independent line.


2. Nf3 (and to a lesser extent, 2. g3) offer similar transpositional options.