THE SUBJECT OF A SENTENCES MAY BE WHAT PART OF SPEECH
What about nouns and pronouns, including proper nouns?
Here are some excellent websites that will help with basic grammar questions like yours:
Click on TERMS and find what you need.
Lots of good information in various places here.
Use the list on the left as the table of contents to find what you need.
Click on the menu for Word & Sentence Level and then choose whatever grammatical/usage item you need information on.
Under the details, click on the area you need information on and go from there.