Major meanings of 'a':

• a = one
• a = each
• a = every
• a = certain
• a = any

a = one (is always singular countable)
When the NOUN is countable singular, 'a = one' in meaning. Here, function is also the same.
Example: I wish to have a(one) bicycle.

👉🏽Before a number, we can use 'a/some'
Example: A one is a number (any vowel-sounded letter with the same sense 'wa' sound use 'a' before the word)

👉🏽Some + number = not the exact number
Example: Some hundred boys are required to pick up the litter.

👉🏽'a/some' with adjectives
It is used before nouns of quantity (uncountable) or nouns of numbers (countable)
  ➡️ a/some precides an adjective before a noun

  ➡️ a + adjective + noun (when the noun is countable singular)    Example: A good student is always rewarded.   ➡️Some + adjective + noun (when the noun is countable plural)    Example: Can you call me some good students.
👉🏽The + Good = noun
If so, the + adjectives will represent a class/type
Example: the poor, thr rich, the needy, the down trodden, the down caste, the untouchables, the bad, the wicked, etc.

  ➡️some + adjective + noun (when the noun is mass noun, i.e., uncountable nouns derived from material nouns which can be used as countables).    Example: I need some good sugar.   ➡️If an adverb (as, how, so) precides the adjective before the noun, use 'a' before the noun.
adverb(as, how, so) + adjective + a+ noun    Example: India has so pleasant a summer.

a = each/every (in meaning)

  'each' is used before a countable singular but has the sense of collective plural.    Example: Each student should bring a pencil.
  or, A student should bring (each) a pencil (each).   'every' is always used before a singular countable noun.    Example: Every student should do his studies in time.
  or, A student should do his studies in time.

a = any (in meaning)

 It is singular/plural + countable/uncountable.
  Used before singular countable noun
  Used before plural nouns
  Used before uncountable noun (may be quantity or not)
+ in an affirmative/positive sentence.
  any => whoever, whatever, whenever, whichever, wherever.

  Example: (i) Which king of saree do you want?
  Any (saree) = whichever     (ii) Any student (= whoever) can answer your question. Any:   I shall come any day to your place.
I + shall = simple future. Some:   I will come some day to your place.
I + will = certainty, determination, promise, threat.

With languages, you are at home anywhere
- Edmund De Waal