When the word immediately following it begins with a vowel sound. Note that, beginning with a vowel sound is not the same thing as beginning with a vowel. "Honest" begins with a consonant but the "h" is silent, so the first sound is a vowel sound ("o"), and you would therefore use "an" ("an honest man"). On the other hand, "uniform" begins with a vowel, but the "u" is pronounced with a "y" sound (a consonant sound) and therefore, you would use "a" ("a uniform").
Note that an "h" at the beginning of a word is not always silent, and when it is not, you would use "a", as in "a historical fact". Similarly, a "u" at the beginning of a word is not always pronounced with a "y" sound, and when it is not, you would use "an", as in "an unfit mother".
Instead of using the word challenging you can use the word gifted, special, special gifted.
Will, when, why, it, can
or
Myself
I have a stomach ache.
mixture
are
used to be
Answer.
is, it, in the past.
no "It" is not used instead of he and she...
Use the word "how" when asking "in what manner". Use the word "why" when asking "for what reason".