While reading "Harry Potter and the Philosopher's Stone", I came across the next line, said by Hermione Granger:
'If they hadn't found me, I'd be dead now. <..>
But, according to the rules, the sentense should have been like:
If they hadn't found me, I would've been dead by now.
Am I wrong? What is the difference between these two sentences?
Both are fine.
"I'd" is a contraction of "I would". Similarly "Would've" is a contaction of "would have". There is no way to combine all of these three words, so in each of the two examples, although they look a bit different they are almost the same. The difference is tense.
"If they hadn't found me, I would've been dead by now".
"I would have been" is conditional and in the past. The addition of "by now" only means that the death would have occurred before now.
"If they hadn't found me, I'd be dead now"
"I'd be dead now", or "I would be dead now" is conditional but in the present. In both sentences, "dead", is a present state, but follows a death, obviously. Although the conditional part of this statement is about the past ("if they hadn't found me"), the statement about being dead as a consequence is clearly in the present with the use of "now" (as opposed to "by now").
The rules you were taught are horribly incomplete.
The reason she says
"If they hadn't found me, I'd be dead now"
is that in that sentence she's talking about two different time frames. They found her in the past, and she would (hypothetically) be dead in the present.
The rules they taught you only apply when the if clause and the main clause are in the same time frame, that is, both in the past or both in the present. The second conditional is for when both events are in the present or future, and the third conditional is when both events are in the past. If you have an unreal conditional where the clauses have different time frames, you conjugate one clause according to the second conditional, and the other according to the third conditional.
