The word father or dad is a common noun when talking about a man who has children. A common noun is never capitalized.
My brother Sam just became a father.
My brother Sam is now a dad.
Bill’s father is an engineer.
Bill’s dad is an engineer.
Our school is hosting a father-daughter dance next month.
Our school is hosting a daddy-daughter dance next month.
If you are writing directly to your father, then the word becomes his name and you must capitalize it.
Dad, I hope we can get together over the summer.
Happy birthday, Dad.
Dear Father, I’m glad we could spend the week together.
The word father or dad is a common noun when writing about a person. The words father and dad become proper nouns when used in a direct address to the person. Proper nouns must be capitalized.
▶It’s your turn. Decide if a word in this sentence must be capitalized. I’m excited, dad, to see you next week. Did you find a proper noun in here and capitalize it? Good for you!