1. I understand the/a variable noun is uncount when use in a general sense and is a count noun when it refers to a particular instance of something. OK, and I think they can sometimes be used interchangeably like some of these below. Can you check if they can be used interchangeably?

1. exposed to sufficient and good quality input(s)

2. repeated exposure(s) using innovative method(s)

3. activities for effective writing practice(s)

4. creating diverse environment(s)

2. I understand the word "vocabulary" is also a variable noun and a reference book indicated/noted that a certain expressions of quantity can be used with it. Can we said like to mean nearly the same thing?

a lot of vocabulary

a lot of vocabularies

3. I was looking at this sentence from my teenager friend's Collins/Cobuild Compact English Learner's Dictionary and has been wondering why they haven't put the underlined nouns in plural. When I see the possessive "their" I tend to think of it as being followed up with a plural noun, not a singular noun. Help.

An adjective is a word which is used to tell you more about a person or thing, such as their appearance (why not appearances), color (colors??) , size (sizes??), or other qualities.
their can be used as a singular. It's often used this way when his, her, and its all seem unsuitable. Below, we see that a person can be a man (his) or a woman (her), and we know the person is not a thing (its). With multiple choices like this, we often use their instead.

... to tell you more about a person, such as [their / that one person's] appearance, ...

Also, even when their does refer to more than one person, remember that more than one person can own one thing together, so a plural noun is never required after their:

John and Susan own a home. It is their home.

Hi Beleiver,

Questions like the ones you have sometimes are not possible for us to answer without a full context; meaning preceeding text or conversation. For instnance: "input" can be countable or uncountable.

A Dolby Surround Sound amplifier has many audio-visual inputs at the back for many types of media . Countable

Thank you for your valueable input. Collective noun/ uncountable
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 CalifJim's reply was promoted to an answer.
good quality input is the better choice; inputs is possible.
exposure is better; exposures is possible.
methods only.
We'll need more context for practice. It could be either one. The phrase is a bit strange.
diverse environments only. diverse makes it plural, similar to several, which also requires the plural.

a lot of vocabulary does not mean the same thing as a lot of vocabularies.

First meaning:

I have [a lot of vocabulary / a lot of words] to learn before tomorrow's English class. (Here vocabulary is short for vocabulary items.)

Another meaning:

I don't have a large [vocabulary / store of words / inventory of words] in English yet, but my vocabulary is growing. (Here you could say a lot of vocabulary, but it's not idiomatic. Here we say large vocabulary.)

Another meaning:

There is a good [vocabulary / listing of words] in the back of this book.
The [vocabulary / set of words] used for mathematics is different from the [vocabulary / set of words] used for biology. These are two different vocabularies. If you consider the many different sets of words used in the various sciences, you realize that there are a lot of vocabularies.

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Thank you very much.

What factors are involved in determining when to use a "count noun" version or an "uncountable noun" version?

Is it any of these or both?

1) context of the sentence(s)

2) specificity as to how the noun is being used or seen as -- single items or a collective item?

I am asking this question because some variable nouns are hard to discern as to how they are to be perceived in sentences. Do you have some ideas as what can anyone do to clearly distinguish which aspect/part of their variable nature is being played out in any given sentential situation? The variable nouns like input and exposure are sometimes hard to place them? Should I keep looking at the contexts of the sentences that I am using and at the same time, ponder over their applicational intention to decide which noun, countable or uncountable, to use?