Final answer:
The subject in the given sentence is 'a man' which is a noun phrase functioning as the subject.
Step-by-step explanation:
The subject of this question is English.
The sentence 'I saw a man was running across the road' is an example of a simple sentence in English. The subject of the sentence is 'a man', which is a noun phrase functioning as the subject.
The subject is the person or thing performing the action or being described in a sentence. In this case, the subject 'a man' is doing the action of 'running across the road'.
Learn more about subject in a sentence