When the first letter of the next word starts with a vowel or the letter H.
But of course there are exceptions.... a humble opinion.
INSTEAD
you can use or , otherwise, or rather than.
what can i use instead of glyerin in making tootpaste
You Can Use Paper Bags Instead Of Plastic
We could use leather instead.
In this situation, you should use "what" instead of "which."
Will, when, why, it, can
For thickening soups, stews, and gravies, you can use flour instead of cornstarch.
You could not use lightning instead of batteries.
Yes, you can use butter instead of margarine in this recipe.
Yes, you can use ghee instead of butter in this recipe.
Why did they use papyrus instead of PAPER? It's all they had.