Salut Bublinka!
Est qu'est-ce que c'est, le Subjonctif Imparfait? plût est 3e personne du singulier de l'imparfait du subjonctif. (plut - traduction - Dictionnaire Français-Anglais WordReference.com)
"Dans cette lettre il n'était pas une ligne qui ne plût à Thérèse."
Well, I have now to write in English .
This is written language. I am learning French too, but as far as I know, this is formal negative structures consist of ne without
pas or any other negative word. There are two forms:
ne explétif and the
ne littéraire. Dans cette lettre il n'était pas une ligne qui ne plût à Thérèse. -
There was no single line that Thérèse didn't like. (= she liked every line of it)
Ne Expletive is not negation, but it has anaphoric meaning. I'll use their example (site About.com, link above), but how my book explains it.
Elle a peur qu'il revienne.
She's afraid that he'll come back.
(She's afraid that he may come back)
Elle a peur qu'il ne revienne.
She's afraid that he will come back.
(She's afraid that he may not come back.)
Elle a peur qu'il ne revienne pas.
(She's afraid that he won't come back.)
Hope it helps.