I was about to make a comparison of this and the series but I learned that this was made before the series. So I’ll just give an overall plot of the movie as well as some review about it.
The story started with Tomoya having a dream ( or rather, a knightmare). The entire story is told by Tomoya’s point of view. It has the most important events in the series and on After Story. It revolves largely on Tomoya’s recurring dreams, family problems and eventually depression.
Even if they changed tiny details in the movie, they didn’t spoil the story’s theme. The important theme about family and friendship are still there. There are some characters that were given much exposure here than in the anime (like Yoshino-senpai..^O^) and vice versa. The animation is still good but compared to the series, the latter is still better. The music didn’t change that much. It’s almost the same.
So if you haven’t seen any of the Clannad’s sereis, then, better start with this since it gives you the overall background of the series.