Do you know that you should use a vowel if the word next to it starts in consonant? (with excemption)?

You use 'an' if it precedes a word beginning with a vowel or mute h (hour). Use 'a' if it precedes a word beginning with a consonant, or before u or eu when they sound like 'you'. "An F" is correct, as is "an S." The NAMES of these letters, and some other consonants, begin with a phonetic vowel sound (eff, ess). "A U" is correct because it is pronounced 'you'. The English often use 'an' in front of 'h' sounds that we pronounce, but since they often pronounce them muted or nearly so, they're still following the rule.