Help me identify this fantasy series!

Help!! I don’t remember the name of this awesome fantasy book series!!

I listened to a fantasy series on audio a few years ago and wanted to listen to it again but I couldn’t find it again. Please help! I have given a full description of my memory of the books. I can give more if needed.

At the beginning of the book the main character who is male and his dog go hiking and fall though a gap between worlds. The dog turns into a horse. The man proceeds to save 2 female characters from some highway men and finds that he is very adept at fighting but can’t understand the language of the land. One of the female characters (who also happens to be the princess) then casts a spell on him so that he can understand them and they find out that he has a very strong magical ability. The other female character is a water nymph. Over the course of the first book she and the protagonist fall in love.

In the second book the characters get sent back to the protagonist’s home world where they have to find a way back to the world of the other characters. They learn that magic is bound up because of the presence of a dragon and they work to find the dragon and use his abilities to get them back to the other plane.

At the end of the series the protagonist uses his magic to conjure a dragon to fight a demon world wrecking monster. But it’s all a ruse to stall while he waits for a real dragon to get there to fight for him.

WHAT IS THE NAME OF THE SERIES!?! I can’t for the life of me remember any names of characters or the series. Thanks for helping!