Make sure to see Titles, from Wednesday’s post regarding capitalization of Titles.
Sentences are much easier than Proper Names or Titles.
The first word of every sentence is always capitalized. This is self-explanatory and universally understood.
Also, capitalize the beginning of a sentence when it is part of a quote: Think complete sentences.
e.g. The teacher said, “Your use of uppercase letters is improving.”
However, if a phrase fits into the larger sentence, it does not require capitalization:
e.g. The doctor told us the nurse would “be here shortly,” but she never came.
This is the end of Capitalization. Thank you Thoughtco.com