Do I need to file a tax return even I have only salary income?
Yes, if you had more than an employer or 2 different income sources in a year, you must file a tax return by the end of May.
Otherwise you may get a tax surprise in 2 years wherever you are, and will be a defaulter even you never know this situation since this dunning letter will could be delivered to your previous year with more than 20% penalty.
So just file by the due date to avoid such a high rate penalty but just pay a late payment penalty.
If you had such a tax arrears, your application of Visa extension can be rejected without a warning.
More questions, please feel free to send me an email to firstname.lastname@example.org