When to use a or an
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THE DIGRAPHS OR DIPHTHONGS
The digraphs (or diphthongs, as they are commonly called) æ and œ are not in as much favor as they have been for the true rendering of Latin and Greek words and their derivatives. Aeneid, Aeschylus, Caesar,[1] Oedipus, mediaeval, etc., are so written now by many classical scholars. In early English names like Ælfred and Cædmon, and in French words like manœuvre, the digraph should be retained.
A OR AN
It is a good general rule to use an before a word beginning with a vowel sound, or in which the initial h is silent, and to use a before a word beginning with a consonant or a consonant sound, or with a vowel preceded by a strong aspirate. The few exceptions cannot be classified.