Use "a" when the next word does not start with a vowel sound. Use "an" when it does...
a very exciting time...
An egg for breakfast...
Remember that the sound of the word following the article (a or an), NOT the spelling, determines when to use "a"or "an."
She earned an M.A. in English.
He has a yellow hat.
CNG can be used instead of petroleum
One alternative word that can be used instead of "they" is "them."
used instead of other elements for what?
The pronoun "one" can be used instead of "we" in the sentence.
list of items where items are being used instead of minerals?
But may be used - instead of however, in most contexts.
instead of being that
An alternative word that can be used instead of "we" in an essay is "one" or "individuals."
rather, preferably, alternately
Instead is an adverb.
ancient greece who was the doctor who used sciense instead of magic
Olbas oil is used instead of wrights coal tar fluid.