Posted: Mon Sep 15, 2008 10:44 am-
You can't keep it if you don't plan on staying. For one thing a claim of permanent residency under the tax treaty means exactly that - you permanently reside in the US. Which means you must file a US tax return every year as a resident. If you carry on living in Canada you will be subject to dual taxation.
You should have filed a 1040 for 2007, in fact you should have filed dual-status, meaning a 1040NR for the part of 2007 you weren't there.
If you don't file tax returns it puts your PR status in jeopardy.
The only way around it would be to file a 1040 in the US and file a non-resident T1 in Canada, however this is impossible if you don't sever residential ties to Canada as the CRA won't believe you.
To maintain your PR status if you are outside the US you have to file USCIS Form N-470, but this can only be done in limited situations (e.g. you work for the US Govt.), from the sounds of it you wouldn't qualify.
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Steve.