偷税 is a separable verb (离合词) meaning 'to evade taxes'. The two characters can be split: the verb 偷 can carry aspect markers like 了 or 过, or a measure word, while 税 remains as the object. For example, you can say 偷了税 ('evaded taxes') or 偷过一次税 ('evaded taxes once'). This splitting is common in real Chinese and you need to use it correctly when talking about the time or frequency of the act.