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If we were talking about the importance of energy and that even machines need it what should we say?

What's the difference between these two sentences:

Machines wouldn't work without energy &

Machines won't work without energy

and between:

Machines couldn't work without energy &

Machines can't work without energy

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    Details...please. – M.A.R. Feb 12 '15 at 18:51
  • Thank you for adding a little more detail. It would help if we saw more context around this sentence, like the sort of sentence that you might write before it or after it. For example, "Our society depends on machines, and these machines won't/can't/wouldn't/couldn't work without energy." Which option seems the best to you? – ColleenV Feb 12 '15 at 22:32
  • You can say any of those. Their meanings are similar though slightly different, but none of them are ungrammatical. – J.R. Feb 13 '15 at 13:33
  • It is often a teacher's responsibility to provide the context when explaining grammar: context is essential for engaging students. – Jon Feb 13 '15 at 16:10
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In the first case, the first statement is hypothetical (using the conditional mood), so considers a hypothetical situation where the machines would need to work, and weren't going to work without energy.
The other is a basic true statement (using the future tense) that simply states that the machines won't work without energy. I should note that this statement doesn't mean they can't work without energy, only that they won't. This is more to do with the idea of free will: consider a person who is able to work without biscuits but refuses to.

The second case is a lot more fun. Saying a machine can't (or cannot) work without energy sort of states that the machine couldn't work without energy even if it wanted to. In this case, because machines aren't sentient, it is identical to saying they won't work without energy.

However, the first sentence in the second group is either conditional or imperfect. In the case where it is conditional, it talks about a hypothetical situation like before. If, however, the tense is imperfect, it implies that the machines, in the past, weren't able to work without energy, but now supposedly can.

Note 1: Machines need energy to work. Fact of physics.

Note 2: I have no way of knowing what tense 'couldn't' is in. Normally you'd need to use the context to work it out.

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This serves as a complement to the MMJZ's answer.

Without contexts, the first sentence in the second group could be paraphrased in the following ways:

  1. We all know that machines couldn't work without energy. (It is impossible that machines work without energy.)

  2. We're running out of fossil fuels. Machines couldn't work without energy. (Machines would not possibly work without energy.)

  3. In fourteenth century people didn't know machines couldn't work without energy. (Machines weren't able to work without energy.)

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  1. As other answers have said the first sentence sounds hypothetical. The reason this statement is hypothetical is that the machines being talked about do have energy. With context it might read:

Of course, all machines are either connected to an electricity supply or powered by some other energy source. Machines wouldn't work without energy.

  1. The second sentence could either be talking about the future or a present 'habit'. Assuming the idea is to describe a habit that machines have, with context it might read:

It is common to see machines connected to some kind of power source: Machines won't work without energy.

  1. The third sentence is quite similar to the first, this time the emphasis being on hypothetical capability. Machines can function but they only have this capability because of energy.

Machines are capable of performing amazing tasks but of course they need a power source. Machines couldn't work without energy.

  1. The final statement is one of fact about machines' capabilities - true now, true in the past, true always.

It is impossible for machines to operate unconnected to a power supply: Machines can't work without energy.

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