Hi,

i'm resident in Thailand, i have a WP and i'm working for a company as staff.

if i do an extra activity beside my usual job, for exemple if i translate a book, for which i'm paid, for a friend's company which is based in USA ,

then the 1 time amount of the pay check is directly sent to my bank in Europe.

what do i have to declare in my personal income tax in thailand? do i have to declare something?

quite complicate. thanks for help

Unless your WP is specifically for 'translation', to do as you suggest would actually be technically illegal. Your WP only covers you (normally) for 1 x specific job at 1 x specific location, nothing more.

Personally, I'd just have any payment made for that kind of thing paid into my personal account and forget about worrying about taxes and things.

income earned offshore is only taxable if transferred into Thailand in the year it is earned.

For instance, money earned on the 30th of Dec is taxable if transferred in on the 31st of Dec, but tax free if transferred in on the 1st of January.

If the OP wants to declare the money for taxation purposes, there will be a box on his tax return where he can declare the income along side his regular salary. The Thai tax return form is a very simple 2 page affair, and is available in English.