First I need to point out that I have NEVER seen the British Forsyte series, so I cannot possibly compare this movie to it. I have heard that the British series is better and fuller, but I just have no idea if this is true.
Second, this movie has among the strangest casting I have seen in some time! Errol Flynn, well-known as a party-animal and womanizer, is cast as the incredibly dull and loveless Soames Forsyte! And, oddly, Walter Pidgeon plays the nice Jolyon Forsyte. Logically, Flynn SHOULD have played the other role and vice-versa, but perhaps the powers that be at MGM couldn't allow "Mr. Miniver" and "Mrs. Miniver" to have a loveless marriage (roles that Pidgeon and Garson were very famous for). Regardless of the reasoning, it actually worked pretty well.
As far as Flynn goes, this casting was probably a good decision, as his role was by far the most interesting and enigmatic in the film. While Garson and Pidgeon play likable people and Robert Young plays an ardent suitor, Flynn must play a very cold and cynical man. The expression "he knows the price of everything and the value of nothing" is a great description of him--a fool who might be rich but can't appreciate love or things cash can't buy him. It must have been tough for the dashing Flynn to play such an overly restrained type of character and it was one of the best roles of the later half of his career.
Greer Garson essentially plays the stereotypical "Greer Garson" role--you know, the nice and sweet lady you naturally love. And Pidgeon plays essentially the same nice guy role he usually played in the 1940s as well. As far as Robert Young and Janet Leigh, they were important to the plot, but their characters weren't really all that important to the plot--they just helped the story along.
Essentially, the story is about very rich and boring old Soames Forsyte (Flynn) pursuing Irene (Garson) very, very ardently. However, his passion and love seems to completely vanish once he marries her--almost like she's a financial acquisition, not a loving wife. Later, Irene meets her niece's boyfriend (Young) and they somehow fall in love. In some ways this is very, very unconvincing--after all, Irene is nice and the idea of her stealing her niece's beau is a bit hard to swallow. But, considering what a cold fish Soames is, I could certainly understand her jumping at the first chance for passion. I just would have liked the movie a little more if the object of her affection were better defined or if this infatuation seemed a bit more believable.
Anyways, what exactly happens next, I'll not explore further, as I don't want to spoil the film. So how all this is resolved is just going to have to wait until you see the film. However, I will say that the very best line in the movie was the final one given by Irene. Speaking of now ex-husband Soames, she says "I feel sorry for him,...he's a very poor man". What a fantastic line--a marvelous way to wrap up this excellent film.
PS--On a sad note, you might want to compare the Errol Flynn in this film to the same man in movies from just three or so years earlier. I saw this movie as well as OBJECTIVE, BURMA! (1945) and NEVER SAY GOODBYE (1946) all a day or so apart. Despite being made about the same time, Flynn was a lot fatter and puffier in THAT FORSYTE WOMAN due to the effects of alcoholism. Sure he was supposed to be older in this film, but the effects I am talking about were not created by the makeup department. His drinking had finally taken its toll and he looks at least 15 years older just in three years! It's very depressing, really, that he did this to himself--going from a very handsome leading man to a bloated soon-to-be has-been (his films of the 1950s were, for the most part, third-rate messes).