Can should not be used in this sentence. She is worried that John would/may/might leave the house when he marries and she would be left living on her own. (OR She is worried that John would leave her on her own when he marries.) These three modals denote possibility. She is worried that John will leave the house when he marries and she would be left living on her own. Will is more definite, but in this sentence, the difference in meaning is slight. See more here: https://learnenglish.britishcouncil.org/en/english-grammar/certain-probable-or-possible

Hi Joe the modal verb 'might' is used when we talk about a probability - how sure are we that something is happening or not happening? E.g. He might be lost - we're not sure about that. In this case, you could use the modal verb 'could' as well. The modal verb 'can' is used to express an ability. E.g. He can do this - he's able to do this It's used as well to express a permission. The modal verb 'may' is also used to express a permission. The modal verb 'will' is used to express probability or a habit. E.g. He will be lost - we're pretty sure he's lost. Finally, 'would' is also a modal verb that is used to express a habit. E.g. When I lived in London, I would usually eat out five nights a week. In your sentence, I would go for the modal verbs of probability 'might' or 'will', depending on how sure she is about him leaving the house. Kind regards, Karen

Will or might leave (not left). Will is stronger and more immediate: she is frightened that he intends to do it. Might is weaker: she is simply fightened by the possibility.