[peteretep]

That's far too simplistic, sadly. It's perfectly possible to be tax resident in multiple countries - I am tax resident in two, for example, neither of which is the US.

Here's how you try and work out if you're tax resident in the UK:

https://www.gov.uk/tax-foreign-income/residence

There are automatic resident and non-resident qualifiers, but everything else involves a whole range of factors.

[ruirr]

It depends in the country really. In Portugal and Spain it is really that simple, you do not live more than 180 days in a year in the country, you are not tax resident.