"Él no me diría."
Translation:He would not tell me.
No, the conditional is only used for conditional actions that haven't happened yet. This is tricky because in English we use 'would' in several different contexts. The imperfect ''él no me decía" would mean more like "He would not tell me/he didn't use to tell me (habitually/repeatedly)."
In your sentence the 'would' seems more like a refusal to me, so I believe it would still use the preterit: "He told her, but he wouldn't tell me" = Él le dijo a ella, pero no quiso decirme a mí/ se negó a decirme a mí. But maybe a native speaker can chime in on this one?
You are right!
The conditional only has two forms in Spanish: the simple one (diría) and the "condicional perfecto" (habría dicho). Since this verbal form is supposed to speak about a hypothetical situation, we use it in sentences like:
"If he had been rude, I would have told him to shut up" = "Si él hubiese sido grosero, le habría dicho que se callara".
"If he was rude, I would tell him to shut up" = "Si él fuese grosero, le diría que se callara"
So Spanish conditionals are basically used as the English "if + would" sentences, leaving this here in case someone finds it useful.
(Verbs are not my forte, hope this makes sense and feel free to correct me anyone!).
Edit: see @SGuthrie0's reply to this!
Thanks for the comment. I have saved it in my notes.
One slight correction of the English. The English subjunctive would be "if he were rude" "Si el hubiese sido grosero." (or more literally, "Had he been rude...." (if he had been rude).
Saying "if he was rude" (Indicative) is saying ("If it is true that he was rude...") then I would,,, -- both clauses are statements of fact, not doubt or uncertainty. Both would be English indicative
"If it were true, that he had been rude" = "If he were rude.." These are English subjunctive. Conditions contrary to fact.
The English translation that Duo is giving me is "He'd not tell me." That is a poorly constructed sentence and I don't think that it's grammatically correct. I translated the sentence as "He didn't tell me" which was marked wrong so I am not sure what the correct answer is supposed to be.