What are the trickiest rules in English grammar pertaining to commonly used words?

I use electronic communication in liasion with othjer agencies.  keep record and communicate

We believe that we use the article “a” before words that begin with consonants and “an” before words that begin with vowels. But that isn’t the rule.

The real rule is this: We use the article “a” before words that start with a consonant sound and “an” before words that start with a vowel sound. For example, He has a unique point of view on the subject and talked about it for an hour. The “u” in “unique” makes the “Y” sound—a consonant sound—therefore you use “a” as your article, while the “h” in “hour” sounds like it starts with “ow”—a vowel sound.

The first thing that comes to my mind is formulating the grammar rules into chemical-like formulas!