Some words can act as both countable and non-countable, depending on context. Would you like some salad? The salad is in a large bowl, and you are being asked whether you would like some. Would you like a salad? You are being asked whether you would like to order a salad with your meal. Usually a person orders only one. I hope this helps.

Uncountable nouns don't have their own natural unit of measurement. When you see uncountable nouns used as countable nouns, it means the noun has been "portioned" or put into units in some way. This won't work automatically; the "portion" must be clear from the context. In the case of salad, when you see "salads", this means either: types of salad, separate bowls of salads, or separate portions of salad.