Learn English – Can we really count ‘fireworks’

The quick answer: In most contexts, these two words are effectively synonyms, especially when discussing possibility, e.g. "(Can/Could) you [perform some action]?".

Longer answer:

There is a somewhat subtle difference in these two phrases that becomes a lot less subtle in different contexts. Let's take the words one at a time:

• Can deals with actual, literal ability to perform an action. When you ask if someone can explain something, you are literally asking whether they are capable of explaining it. Edit: It is important to note that, as @J.R. has pointed out, this is by no means the only use (or even the most common use) of the word can or the phrase can you .... This construction is often abused to mean effectively the same thing as could you..., i.e. would you be willing to..., etc.

• Could deals with the possibility of performing an action. It is definitely a subtle difference, but basically, when you discuss possibility, you generally are aware of their ability to do it but are tentative about their willingness or availability to perform the action. This is by no means an exact definition.

It is also worth noting that the word could is actually the past tense of can. In English, using the past tense form of a word in this way is often meant to indicate tentativeness.

I wanted to talk with you.
I wondered if you would be available this afternoon.
I thought it might a good idea.

Quote: My dictionary says prey is an uncountable noun.

I would say take such indications as a general hint. Nevertheless a writer can choose whether he wants to use that word as a collective noun with a singular verb or as word with plural meaning in the sense of all the animals that are prey. In this respect - countable or uncountable - English is flexible and tolerant. You will find this flexibility with almost all nouns that are collective nouns for groups. You can use "police" as a singular when you think of the whole of that organisation and you can use it as a plural noun when you think of the lot of people in the police force.