“A.D.” does not mean “after death,” as many people suppose. “B.C.” stands for the English phrase “before Christ,” but “A.D.” stands confusingly for a Latin phrase: anno domini (“in the year of the Lord”—the year Jesus was born). If the calendar actually changed with Jesus’ death, then what would we do with the years during which he lived? Since Jesus was probably actually born around 6 B.C. or so, the connection of the calendar with him can be misleading.
Many Biblical scholars, historians, and archeologists prefer the less sectarian designations “before the Common Era” (B.C.E.) and “the Common Era” (C.E.).
Traditionally “A.D.” was placed before the year number and “B.C.” after, but many people now prefer to put both abbreviations after the numbers.
All of these abbreviations can also be spelled without their periods.
You should use “an” before a word beginning with an “H” only if the “H” is not pronounced: “an honest effort”; it’s properly “a historic event” though many sophisticated speakers somehow prefer the sound of “an historic,” so that version is not likely to get you into any real trouble.
If the word following begins with a vowel sound, the word you want is “an”: “Have an apple, Adam.” If the word following begins with a consonant, but begins with a vowel sound, you still need “an”: “An X-ray will show whether there’s a worm in it.” It is nonstandard and often considered sloppy speech to utter an “uh” sound in such cases.
The same rule applies to initialisms like “NGO” (for “non-governmental organization”). Because the letter N is pronounced “en,” it’s “an NGO” but when the phrase is spoken instead of the abbreviation, it’s “a non-governmental organization.”
When the following word definitely begins with a consonant sound, you need “a”: “A snake told me apples enhance mental abilities.”
Note that the letter Y can be either a vowel or a consonant. Although it is sounded as a vowel in words like “pretty,” at the beginning of words it is usually sounded as a consonant, as in “a yolk.”
Words beginning with the letter U which start with a Y consonant sound like “university” and “utensil” also take an “a”: “a university” and “a utensil.” But when an initial U has a vowel sound, the word is preceded by “an”: it’s “an umpire,” “an umbrella,” and “an understanding.”
When you turn 360 degrees you’ve completed a circle and are back where you started. So if you want to describe a position that’s diametrically opposed to another, the expression you want is not “360 degrees away” but “180 degrees away.”