Which commandment made Paul aware of all the other commandments?

The commandment 'Thou shalt not covet' made Paul aware of all the other commandments. In Romans 7:7, Paul identifies coveting as the commandment that put him in awareness of the rest. This awareness encompassed all the other commandments. It was the one commandment that opened his understanding.

Les teaches that the commandment 'Thou shalt not covet' is the one that put the apostle Paul in an awareness of all the other commandments. He directs attention to Romans chapter 7, verse 7, where Paul addresses this. In the broader context, Les notes that even uncivilized tribes have a moral code, and Paul states in Romans that for those who did not have the written law, God put the law in their hearts, with conscience accusing or excusing them. But the specific commandment that brought Paul to awareness of the rest was 'Thou shalt not covet.'

Romans 7:7

What shall we say then? Is the law sin? God forbid. Nay, I had not known sin, but by the law: for I had not known lust, except the law had said, Thou shalt not covet.

Why did Paul use 'thou shalt not covet' as the primary example of the law?

Paul uses it because coveting is the trigger behind every violation: no one breaks any of the Ten Commandments unless he covets first. The command exposed lust or desire, and sin, taking occasion by it, produced all manner of immoral thinking, making it the clearest example of the law's function.