Q:

'Preclude' vs 'Exclude' vs 'Prevent'

I'm trying to really understand how the word preclude differs from either exclude or include. Merriam Webster defines preclude as

to make impossible by necessary consequence : rule out in advance

Which sounds a lot like either prevent or exclude. Does prevent block a future action from happening, while preclude eliminates the possibility of that action altogether?

A:

You are correct that there is some overlap between "preclude" and both "prevent" and "exclude". They may sometimes be almost interchangeable, but certainly not always.
I would say that preclude is used to indicate that one event or circumstance prevents another event or circumstance from happening. It indicates cause and effect.
You've referred to the definition in Merriam-Webster, but did you also look at the usage examples that it gives:

She suffered an injury that precluded the possibility of an athletic career.  

i.e. The injury prevented her from pursuing an athletic career.
She couldn't pursue an athletic career because of the injury.

Bad weather precluded any further attempts to reach the summit.  

i.e. The bad weather prevented them from making any further attempts to reach the summit.
They couldn't make any further attempts to reach the summit because of the bad weather.

The secret nature of his work precluded official recognition. (Example from ODO)  

i.e. His work could not be officially recognised because it was secret.
One dictionary definition seems quite explanatory:

Remove the possibility of; rule out; prevent or exclude; to make impossible.

and, helpfully, gives an example of not precluding:  

It has been raining for days, but that doesn’t preclude the possibility that the skies will clear by this afternoon!

i.e. it could still be a clear afternoon even though it has been raining for days.
Using not preclude in this fashion is also fairly common.
So:

Yes, it does mean "prevent", but the word is used in a slightly different way.
Yes, it does mean "exclude", but "exclude" does not carry the implication of cause and effect.

