How to use "warn" in a sentence

Israel Schwartz was Jewish, so perhaps when the first man saw him, he shouted 'Lipski' to warn him aggressively to go away

But what could he do? He could not warn M

I tried to warn you before."

So when my sister-in-law phoned at two in the morning, asking me to come over, but first to warn the police that she had just killed my brother, I spoke in my usual calm manner.