Why 1st command no working when my 2nd command have some typo

The reason the missing closing paren in the console.log line causes the previous line to not run is that the in-browser implementation of JavaScript has to parse or compile the code first (I don't know this stuff well enough to know which is the correct terminology), before it can run the individual lines of code and this line of code cause the compiler to break, so it never gets to the point of running the individual lines. Note that this would happen whether or not the console.log error was on the 2nd line, or the last line.

For the 2nd question, Guil didn't say that document.write replaces everything. He said that when we use document.write from from the console, it overwrites everything. The reason for that is that the document is closed, and when you do a document.write from the console, it opens the document, which wipes everything out, then writes to the document. When you are writing to it with the javascript the way that we are doing, the document is still open, so it performs the write operation on the document when the script is loaded.