I am Canadian. I stayed 6 months in US and left back to Canada. I want to go back to USA but I haven't been outside of the US for 6 months yet.
How do I do that? I know that when you are in US, you can apply for an extension. Can you do that from outside the US? And how does the extension work? Is the extension valid from the time it is approved or from the time I left US?
You can go to a US port of entry and simply seek entry as a visitor at any time. There is no rule saying you cannot be admitted to the US before you have been outside for a certain length of time. In theory, you can stay in the US for 6 months, leave the US, and then 1 minute later turn around and seek to enter the US again. And it is legally permissible for the officer to admit you for another 6 months. And there is no rule that says you can't do this again at the end of that stay, and so on.
Each time you seek entry as a nonimmigrant, the officer can deny you entry if they determine that you failed to overcome the presumption of immigrant intent. A long stay followed by a visit very soon after might increase the probability that the officer determines that you have not overcome the presumption of immigrant intent. But this is a subjective determination, which can vary from case to case and officer to officer. There is no guarantee that you will be able to enter after a long period outside the US, and there is no guarantee that you won't be able to enter after a short period outside the US.
I know that when you are in US, you can apply for an extension. Can you do that from outside the US?
No. You can apply for extension of status using USCIS form I-539, but the request would automatically be cancelled once you leave the US (since there's no more status to extend). You cannot extend your status while outside the US since you have no status.
And how does the extension work? Is the extension valid from the time it is approved or from the time I left US?
The extension is valid as long as it is approved (you receive a notice of approval from USCIS) and it is valid until the date the approval notice specifies. If the request is denied then your status expires at the original expiration date of the status. You may stay in the US while the request is pending, but if your status expires and the extension request ends up being denied - you risk ending up in an overstay status.
