You have an old car that has lots of engine problems. Your sister is trying to convince you to sell it, but you don't want to. One reason you don't want to sell it is that you don't think you can make a lot of money by selling it to someone. You say:
I doubt I'll get much for it.
"I doubt ___" means "I don't think ___":
I doubt I'll be able to make it.
I doubt the weather will be any better tomorrow.
But you can't substitute "I doubt" for "I don't think" in sentences like this one:
I don't think he should do that.
In this sentence, "I think" is used to express your opinion. "I doubt" can only replace "I don't think ___" when it's being used to make a guess.
The phrase "I doubt ___" carries a greater level of disbelief than "I don't think ___". In other words, use "I doubt ___" for things that you really don't think are true.
In questions and negative sentences, "get much for ___" means to make a lot of money when you sell something. People usually use this phrase to talk about selling their used items.
Did you get much for the Toyota?
I didn't get much for it.
When you pronounce the phrase "get much for ___", the stress is on the word "get".
The example at top is a negative sentence because "I doubt" is negative. You don't use "get much for ___" in positive sentences. Instead, say something like:
I actually got quite a bit for it.
He says he got $200 for the old one.
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