1/ For future actions dependent on the result of another future action or event, where there is only a small possibility of the conditions for the action being satisfied.
eg: If I won the lottery, I would stop working.
2/ For imaginary present actions, where the conditions for the action are NOT satisfied.
eg: If you phoned home more often, they wouldn't worry about you. (The conditions are not satisfied because the person does not phone home, so they do worry.)
TO BE: In Standard English this verb can take the 'were' form for all persons in the If clause.
eg: If I were you, I'd tell her.
CONTRACTIONS: 'Would' and 'had' are contracted to 'd; the way to distinguish them is simple because 'would' is always followed by a Base Form and 'had', as an Auxiliary Verb, is followed by a Past Participle.
eg: I'd tell her. 'Tell' is the Base Form so it means 'I would tell her'
I'd done it. 'Done' is the Past Participle so it means 'I had done it'
Conditionals; Modal Verb; 1st Conditional; 3rd Conditional; Zero Conditional; Verb Related Article English Conditionals - Read up on the English Conditional and discover how they are used to talk about possible or imaginary situations