I found this question on Quora.
Here’s my answer:
Every line matters.
In your code every line matters. There are no lines that do nothing. (If there are, you should delete them.) So if your code doesn’t do what you expect, go back and see if you didn’t understand one of the lines.
In the book or lectures you’re learning from, every line matters. If you don’t understand a concept, go back and look for a line you skipped.
Programming is easier than in the Old Days™. But this rule still abides. Every line matters.