La Zone d’entraide est en travaux.

Bonsoir !
En fait, il existe une différence entre le real conditionnal(first), unreal conditionnal(second) et le third conditionnal)
Le real conditionnal est composé de la particule if et du simple present.
ex : ​If I go home for lunch,​I eat pizza with my sister. (ça c'est quand c'est un fait avéré)
Sinon, il y a le first conditionnal, qui est utilisé avec le futur simple ​: If there is too much noise, then we will leave.
Il y a aussi le unreal (qui est quelque chose qui n'arrivera pas, c'est un temps supposé). Il est constitué du simple past + would + base verb (verbe à l'infinitif).
Il y a aussi le third conditionnal, qui est ton cas.
If you had told me you needed a ride, I would have left earlier. (Grammarly)
If I had cleaned the house, I could have gone to the movies. (Grammarly).
Ici, comme tu peux le voir, il y a la proposition avec if composée de had + participe passé du verbe, suivie de would/could + have + participe passé.
Reprenons donc tes exemples ensemble :
They ( help )….. you if they could have.
They + would + have + participe passé de help
They would have helped you if they could have
Pour le negative, c'est la même chose, mais tu mets -not après would.
He would not would have written unless he was lonely.
He would not have written unless he was lonely
If I had known you weren’t coming, I would not would have baken.
If I had known you weren’t coming, I would not have baken.
If + had + pp + would +(not) + have + pp
Ta dernière phrase est correcte, il te manque juste un pronom ! ;P
If (pronom) hadn’t missed the train, he would have arrived.
Voici un lien qui pourrait t'aider : https://www.grammarly.com/blog/conditional-sentences/
Bonne révision ! :D
Sarah G