an atom vs the atom [duplicate]

I can’t for the life of me figure out where to use a and where to use the — and where there is no article at all. Is there a simple rule of thumb to memorize?

The standard rule you always hear:

“If a person knows which item you are talking about then use "the"

. . . doesn’t clear things up for me, as I have no idea whether or not they know.

Solution 1:

Well, if you insist on the rule being simple, here you are:

• a = some, any
• the = this, that

Two simple examples. Note that you just wrote "...if a person knows which item you are talking about...". You didn't write "...if the person knows...". And that's correct, because you are not pointing to this or that person, you are talking about any person in general.

On the other hand, my answer starts with "if you insist on the rule being simple". That's because you asked for a rule (= any rule), and I am now talking about that rule. We are talking about the same thing.

Now, I can't think of a (= any) simple rule of thumb when not to use an (= any) article at all, but here are some suggestions:

• Don't put an (= any) article before a (= any) noun if the (= that) noun is preceded by:
  • a number
  • a possessive adjective ("my", "his", "our"...)
  • a "no", "some" or "any"
  • a "this", "that", "these" or "those"

Examples:

• Give me a chair! (= any chair you like)
• Give me the chair! (= this chair)
• Give me that chair! (no article, you already specified which chair you mean)
• Give me my chair! (no article)
• Give me five chairs! (no article)
• Give me some chairs! (no article)
• Give me the chairs! (= these chairs)
• Give me these chairs! (no article)
• Give me a reason to hit you! (= any reason will do)
• Give me no reason to hit you! (no article because of "no")
• Give me no reason to hit you with a chair! (= any chair)
• Give me no reason to hit you with the chair! (= this chair)

Solution 2:

There isn't a simple rule, as the definite and the indefinite article are used in different cases.

• Eat an apple before going to the cinema.

  The sentence is not referring to a particular apple; it can be a random one.

• Follow the President.

  The sentence is not referring to a random president.

The difference is not between talking of something the other person knows, or not.
In the second example, the person I am talking to could know all the presidents present in the meeting, but he will understand to which president I am referring.

• Albert taught himself to play the violin.

• Worry about the future.

  In this case, the article is used to make a generalized reference to something rather than identifying a particular instance.

• They placed the African elephant on their endangered list.

  African elephant is referring to the whole species, not to a single elephant.

• She is a McFry.

  The sentence is referring to a member of the family McFry.