One employer or several?
With several employers, total employment income and income excluding the first employer may be subject to different threshold tests. Review the legal employer structure where group-company changes, transfers or employer changes occur during the year.
Collecting payroll statements, cumulative tax-base letters and withholding figures in January makes the March filing review more reliable.
Foreign salary and deductions
For foreign-currency salary from an overseas employer, the employer's position in Turkey, source of payment and nature of the work matter. Do not assume an automatic exemption.
If a return is required, separately check documentation and percentage limits for deductions such as education, healthcare, insurance and donations. Filing is about documentary discipline as well as tax.