Why do we say "I would appreciate it if you paid in cash," but not "I will appreciate it if you pay in cash"?
In the dictionary, I found this example (Source):
I would appreciate it if you paid in cash.
Clearly, this is the conditional sentence, type 2 which expresses something that is impossible in the future or at the present. For example the following sentence expresses something that is impossible in the future or at the present:
If I won lottery, I would buy a 10 bedroom house.
So, let’s say you want to buy a newspaper at a shop; you give your debit card to the seller; and the seller says:
I would appreciate it if you paid in cash.
Does the seller think that the chance that I can pay in cash is impossible?
Why can‘t the seller use the following conditional sentence, type 1, meaning something that is possible in the future or at the present?
I will appreciate it if you pay in cash.
This way he says that he thinks that the chance that I can pay in cash is possible.
So, which structures do native people use?
I would appreciate it if you paid in cash.
I will appreciate it if you pay in cash.
Conditional constructions are vastly more complex than the “first, second, third conditionals” teachers employ to introduce them. Now that you are dealing with expressions which do not conform to the ‘canonical’ n-conditionals, you are ready to discard those pedagogical baby rules.
The use of past-form would in the apodosis (consequence clause) does not necessarily, or even usually, imply impossibility. It may express actual past tense; it may express the contingency or non-certainty of the consequence; it may express willingness; it may express a recommendation—or it may, as in this case, primarily express courtesy: a polite indication that the speaker does not require you to pay cash but earnestly hopes that you will do him that kindness.
Expressions such as I would appreciate it if you paid in cash are commonly used to soften a request, to make it less direct, and thus to avoid putting an onus on the person being spoken to. Grammatically, you are right--the speaker is using the conditional incorrectly here. But this is a common colloquialism that is not restricted to English. In French, you'll hear things like "Je voudrais aller" (I'd like to go) when, to be completely literal about it, the proper response is "I want to go." In your example, the idea is that the speaker is not demanding to be paid in cash, but he is making it clear he prefers (would prefer) cash.
See this: http://dictionary.cambridge.org/us/grammar/british-grammar/politeness Also: http://blog.harwardcommunications.com/2014/07/30/how-to-be-polite/
The difference is not so much in the type of the conditional, but the tense of the verb, specifically future versus future-in-the-past.
For sake of discussion, imagine that the seller is himself imagining the situation a few minutes from when he speaks, wherein the buyer has already paid. At that hypothetical future moment, the seller appreciates that the buyer has paid in cash. The way that is phrased is to use the future-in-the-past tense, or with the word would
Conditional does not necessarily equate to "impossible now"; it can simply represent a hypothetical scenario. In other words, it hasn't happened yet (e.g., "If I won the lottery..." [but I haven't yet], "If you paid in cash..." [but you haven't yet]). Of the two options you provided, native speakers, when speaking correctly, would say, "I would appreciate if you paid in cash." Note the use of "would" in my own sentence ("native speakers...would say"). This is conditional and in this case means "if...then" (if speaking correctly, then they would say). That (if...then, otherwise known as a hypothetical scenario) really is often what conditional represents.