Very good nations to learn the basics are either the underwater nations , Atlantis and Ryleh or Ermor Ashen Empire/Soul gate themes .
Or you could play the Faerun Map with only about 5 Opponents there you have then enough time to expand before you meet your enemies .
When you start with dominions the AI opponents seem strong because you are not prepared to their rushing tactics
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Once you know the basics normal AIs shouldn't be a too big problem anymore
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Other good starting nations are Man , Vanheim , Jotunheim and Marignon because they have very good troops .
Man , Marignon and Vanheim are magically easy to play also .
All in all i would recommend man for starting because they are straight forward + you can summon your first AQs in midgame to learn the use of SCs .