Difference – ‘He Hasn’t Said’ vs ‘He Did Not Say’: Rules and Usage

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.

There are times where I would tend to favor one over the other, but I'm having trouble figuring out a simple rule that dictates which one I would choose. I would describe:

• The aroma of bread, coffee, and wine, but
• The scent of a skunk, a perfume, a flower, or a woman

Aroma seems to be the more natural word for describing blends of food and spice, while scent seems to be more apt for describing the natural smell of a plant or animal.

Also, not too unlike atoms and molecules, scents seem more fundamental, while aromas seem like more of a mixture or blend. So, I might talk about the scent of a particular candle, but the aromas of the candle store.

Still, the literature has plenty of counterexamples, such as the scent of bread, or the aroma of the flower, or the scent of the wine, so it would be a mistake to interpret any of these thoughts as "rules". The two words are largely synonymous, so there's plenty of room for overlap. If someone said that only one of the two words could be used in a particular context, I'd regard that as smelling fishy.