To identify the sentence subject, ask yourself "Who is doing the action?" or "What is doing the action?" The answer will be the subject. In "Birds fly high," ask "Who flies?" The answer is "Birds," making it the subject.
Look for the noun or pronoun that comes before the action word (verb). This subject identification skill becomes easier with practice using simple sentences.
Subject Examples in Simple Sentences
Here are clear examples of identifying subjects in everyday sentences:
"Emma reads books" - Emma is the subject who reads
"The dog barks loudly" - The dog is the subject who barks
"Children play outside" - Children is the subject who play