So Clym finally breaks the news of his engagement to his mom, who flips out.
They have a huge argument. Mrs. Yeobright disses Eustacia's family, Eustacia herself, and Clym.
Mrs. Yeobright finishes her rant with a flourish – "If you wished to connect yourself with an unworthy person why did you come home here to do it? Why didn't you do it in Paris? – it's more the fashion there" (3.5.31).
Clym storms out after that and goes to commune with nature.
Eustacia comes to meet him and sympathizes with him over his mom.
She agrees to get married immediately and Clym shares the happy news that they'll have to live in a tiny cottage at first while he continues to study.
Eustacia makes him promise it will only be for a few months and says her grandpa should be cool with it as long as it's not for too long.
Clym gets depressed after Eustacia leaves.
But he feels like he's stuck now and must make her see the best of it.
Apparently, if your last name is Yeobright, you have a tendency to rush into ill-advised marriages and then regret it soon afterwards.