"dog" is the singular form of a countable noun, and "dogs" is the plural form of the same noun. "Air" is uncountable-- you can't say "one air, two airs". In addition to counting, we will say "a" in front of singular countable nouns, but not in front of uncountable nouns. singular countable: I saw a tree. uncountable: I saw water. BAD: I saw tree. BAD: I saw a water. Sometimes a word will have several definitions, and the singular countable definition might be different from the uncountable definition. experience (uncountable)= knowledge and skill in a particular area="Bob has a lot of computer experience. You should hire him to fix your computer." experience (countable)= a thing that happens="I had a lot of strange experiences yesterday. I saw a man with a banana on top of his head, and I saw a camel walking in the middle of the road." Sometimes, there will be two different countable definitions of a word, and one of them will usually be used less often as a plural than another one. pace (countable)= a step= "he walked three paces" pace (countable, usually singular)=speed="He walked at a slow pace." wood (uncountable)= a building material= "This house is made from wood." wood (countable)= a type of golf club="Bob selected a wood instead of an iron." the woods (countable, always plural)=a forest="The woods were dark and scary." Other times, some words will always be in the plural or singular form. For instance, in America, "math" is always singular, and in Britain, "maths" is always plural. "accounts receivable" is always plural. It is the name of a single department, but it is grammatically plural, and since we use the plural form to talk about a single thing, we will never use the singular form. the metaphorical meaning of "the negotiating table" is always singular.

Thanks a lot!

"Usually" means "most of the time" or "more often than not". Singular noun: One cookie (or two cookies, they are easily able to be counted individually) Uncountable noun: Grass (a field of grass is made up of lots and lots of tiny blades of grass. Although we could count every single blade of grass, we would not say "one grass" or "two grasses" when talking about one or more areas that each have many, many blades of grass, such as a lawn in front of or behind a house. We express uncountable nouns with words or phrases such as "some" or "a lot of" or "a little bit of") Examples: I cut the lawn earlier, and now there is some grass on my shoes. Could you pour me a little bit of water, please? Hope this helps.