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English as a Second Language/Will/would/could - future/past


Dear Shannon,

I read the following:

"Would a firework work on the Moon?" Yes, it probably would. As you know, fire needs oxygen. So you would think it could not work on the Moon. But many fireworks have a chemical mix that contains a source of oxygen so they could still work."

My question: why would/could are used instead of will/can? If I say: "Will a firework work on the Moon?" will be be wrong? What is the difference in meaning, if any?
Also, "so you would think it could not work... so they can still work" why not saying "so you will think it can not they can still work". Why the writer used the past? What is the difference?

Thanks & best regards,
Antoine Ghannoum

"Would" is often used to express conjecture, and "could" complements this tense (as a conjectural form of "can").  Yes, "will a firework work on the moon" is ok, but when you state it in this way, you leave out the conjecture part.

In the second example, it becomes even more difficult to use "will/can" because here we have already established the conjecture of the firework on the moon, and as no one has ever tried doing this, it is entirely a question of supposition/a hypothesis.  The word "will" doesn't work in this case: "so you will think" -- how can we be discussing your future action of thinking in this context?  It simply doesn't work.

So, in some simple cases, you can replace would with will.  However, once we are talking completely in conjecture, that replacement becomes impossible.

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Shannon Cole


I can answer questions related to learning the English and Spanish language.


I have taught ESL and Spanish since 1998 at the university and middle school levels. I am a native of the U.S., and have taught in both the U.S. and Mexico.

I am owner and operator of, an online language school.

Georgia TESOL in Action (1999)

B.A. in Spanish; M.Ed. in Language Education

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