Capital letters are used in written English in the situations which are discussed below. Lower case letters must be used in all other instances.

Capital Letters: Rules and Usage

When are capital letters – also known as upper case letters – used?

1. As the first letter of a sentence
“Do you like grammar?”

2. The first letter of a proper noun must always be in capital letters.  By proper names we mean: the names of specific people, places, countries, languages, ethnic and national identities, organizations, and things such as the days of the week. astrological signs, names of the months, holidays, sports teams. For example:
Albert Einstein, Katy Perry, New York, Chile, Spanish, Asian, American, Microsoft, Apple, Friday, Taurus, February, Labor Day, New York Giants.

3. The  pronoun “I”
Although I could not really afford it, I still bought the dress.

4. Acronyms and Initialisms
Nabisco – National Biscuit Company,  USA – United States of America, AA – Alcoholics Anonymous, N.I.M.B.Y – Not in my Back Yard

5. To add emphasis in a written piece, by capitalizing word or phrase on which you want to place particular emphasis.
“This is NOT a joke.”

6. Family terms such as Aunt Hattie is they are used as part of their proper name.

7. Religious figures
Buddha, Allah, Jesus Christ, Moses

8. Title preceding names (Again, only when this is part of the proper name.)
Mayor Feinstein, Judge Judy

9. Directions that describe the location
Seattle is in the Northwest of the United States.

10. Periods, Eras, Events
Colonial Era
Swinging Sixties

Capital letters are used in written English in very specific prescribed ways.  For all other purposes, lower case letters must be used.