The Editor's Manual
Free learning resource on English grammar, punctuation, usage, and style.
The article “the” is generally omitted with acronyms of proper nouns (NATO, NASA) but used with initialisms (the US, the UK).
Use “a” or “an” before an abbreviation depending on how it is pronounced, not written. If it starts with a consonant sound, use a; if it starts with a vowel sound, use an (“a NATO member” but “an NFT”).
Count nouns have singular and plural forms (e.g., star/stars). Mass nouns are thought of in terms of quantity rather than number of units, and have no distinct singular and plural forms (e.g., milk, excitement, oxygen).
Initialisms are abbreviations of which the letters are individually pronounced (UK, USA, ATM), while an acronym is pronounced as a word (NATO, UNICEF, laser). An abbreviation could also simply be a contraction (Dr., Mr.) or other shortened form (Prof., Dec.).
Accepted guidelines: Provide the full form of an abbreviation at first use. Avoid using abbreviations at start of sentence. Use “a” or “an” depending on how the abbreviation is pronounced. And more.