Learn English – Is it correct to say “There are 5 hepatitis B viruses in his liver”

In general, nouns can be divided into abstract concepts and real-world (concrete) objects. Furthermore nouns can be divided into things measured by quantity or degree, and things measured by number.

As a rough principle, abstract concepts (like love or peace) and nouns measurable by quantity (like water or sand) tend to be uncountable by default. Meanwhile, concrete nouns measured by number (like lions or buildings) are usually countable.

Unfortunately there are far too many exceptions to this to make it completely reliable. For example, one can have many loves in one's life. Fish can be counted by individual animals, or by some uncountable amount of weight or volume ("a full net of fish").

What does seem to work, at least with some consistency, is to find the closest synonymous noun that you know to be countable or uncountable, and use that as a guide. For example, let's say you come across a nifty noun like senescence, which means "the deterioration that comes with age". It's an abstract concept, and probably measured by degree, so you can tentatively assume it to be uncountable. Since you also know that the relatively similar noun crazy is uncountable, you can be pretty sure that senescence is also uncountable. Example:

Even as the greedy king's aging body slipped into senescence and decrepitude, his desire for more life burned ever hotter.

As a counter-example, take an abstract noun like plutocracy ("government by the rich"). Yes, it is abstract, but at the same time you know the noun government is both countable and uncountable. You would not be wrong to assume that plutocracy is the same -- countable when referring to specific instances:

With the current state of campaign finance laws, it would not be an exaggeration to call our current system of government a plutocracy (by proxy), rather than a republic.

And uncountable when referring to the abstract concept:

Plutocracy is the default state of any government that lacks a strong national Constitution.

Your examples mood and freedom can be determined the same way. Mood is like feeling, while freedom is like value. Feeling and value are both countable and uncountable, so you can tacitly assume mood and freedom are the same.

Specific instance:

Mom is in a good mood today.

Yoga gives me a freedom of movement that I don't get from other kinds of exercise.

Abstract concept:

The other captains say that whether you can make it through the Devil's Triangle is entirely up to mood and chance.

What good is bread without freedom?

Nevertheless, because there are so many irregularities, you're likely to have to memorize more than a few, and be corrected when you make mistakes. Even native speakers learn these by reading, remembering, and regurgitating.

For example, suppose I ask you to go to the store with a shopping list on which is "steak". You need to go to the butcher and ask for steak, and you know meat is uncountable, so you assume steak is also -- until you hear the person ahead of you ask for "three large steaks". So you assume it's countable. Then the butcher asks you "how much steak" you want and you realize it can be either, depending on whether you are measuring by weight or by number of portions.

Now you have steak in your set of known nouns, so if you come across another meat-related noun like brisket, you can assume it's similarly uncountable when measured by weight

I need three pounds of brisket for this recipe.

and possibly countable by portion

Dad always cooks a brisket for Sunday dinner.

(Edit) To be clear, when native speakers first encounter a new noun, it's with enough context to specify it as countable or uncountable. Again, to pull out a fairly esoteric example, replevin, a legal term meaning "a procedure to recover unlawfully seized property". Few (if any) who haven't been to law school will know or need this term -- but those who have, will have seen it in context written something like

the law and practice of replevin

or

a writ of replevin

and this is how they will use it, as a phrase rather than a stand-alone term. They wouldn't say "a replevin" or "replevins" -- even if they dictionary says it's OK -- until they see someone else us it that way.

The countable form used in the example, "waters" is poetic in use, and means "streams, rivers, lakes" collectively. Singular use in this sense is very rare, and archaic:

And as they went on their waye, they cam unto a certayne water (from an old translation of the Bible, in modern translations it is "river".)

In modern English this sense is only used poetically and only in the plural.

The plural can also be used for "regions of the sea" The territorial waters of the UK or "spa water" take the waters at Tunbridge Wells. In these senses it is always plural.

As a singular, it means "a serving of drinking water" *I'll have a water and a beer, please. But even in this sense, it is rather rare. "A glass of water" or similar would more common.

In nearly all other use, water is uncountable. Except in the rare cases described above, you don't use the plural "waters" or the countable singular "a water".

The word is unlike "fish" because "fish" is an acceptable plural of "fish". The only acceptable plural of "water" is "waters". The word "water" is singular, never plural.

The expression "a little water" is not a use of a countable noun "a water", it is the compound determiner "a little", which modifies the uncountable noun "water". Consider that you can't say "two little waters", because the compound word is "a little".