Use the definite article ‘the’ if you are referring to 1 specific noun rather than 1 of a larger number.
We need to buy the right paper to achieve the best results.
Developing a right solution will deliver a following benefits.
The choice between ‘a’ and ‘an’ depends on the sound of the noun that follows it. If you pronounce the noun with a vowel sound, use ’an’. If it starts with a consonant sound (including a ‘y’ sound), use ’an’.
We need to present a united front.
I have an hour free now.
I think she made a honest attempt.
This is an one-time offer.
Some guides used to recommend selecting ‘an’ before nouns that are 3 syllables or longer and start with the letter ’h’. This is not grammatically necessary and the ‘rule’ only complicates article use.
The constitutional crisis was a historic moment in Australian politics.