-  on a certain day

The error is in part 4 of the sentence. The correct sentence is:

"The old man in the crowd warned Julius Caesar of the danger of death on a certain day."

Let's understand why part 4 of the sentence is incorrect:

The preposition "on" is used to refer to a specific day or date. However, in this context, the old man is warning Julius Caesar about the possibility of danger on a particular day, not a specific one. Therefore, the article "a" should be used instead of "the" to indicate that it's a certain day but not a specific one.

Here is the corrected sentence:

"The old man in the crowd warned Julius Caesar of the danger of death on a certain day."

Let's understand the meaning of the other parts of the sentence:

• "The old man in the crowd" - This phrase describes the person who warned Julius Caesar. He is an old man, and he is among the crowd of people.
• "warned Julius Caesar" - This phrase describes the action that the old man took. He warned Julius Caesar of the danger of death.
• "of the danger of death" - This phrase describes what the old man warned Julius Caesar about. The danger of death is a serious matter, and the old man wanted to make sure that Julius Caesar was aware of it.
• "on a certain day" - This phrase describes when the danger of death could occur. The old man did not specify a specific day, but he warned Julius Caesar to be careful on a certain day.

Therefore, the correct sentence is: "The old man in the crowd warned Julius Caesar of the danger of death on a certain day."