The full sentence is :-
'Я читала книгу. Она хорошая, но скорее грустная..
Помню, я плакала, когда Татьяна грустная Онегина'
I am guessing that the long form adjectives 'хорошая' and 'грустная' are here instead of 'хороша' and 'грустна' because the latter would sound a bit bookish in conversation?
Also, 'плакала' (imperfective) is used to explain that 'I was crying when...' rather than just 'I cried when...', to emphasise the crying and make it a bit more possible to empathise with?
Am I looking at it right??