The minimum required tread depth for tyres is 1.6 (so during summer) mm and 3 mm during winter conditions during 1 December - 31 March. You’re required to use winter tyres from 1 December to 31 March in case of winter conditions. Studded tyres are not allowed 16 April - 30 September, unless there are winter conditions or if winter conditions are forecasted. Winter conditions means that at least on some part of the road there is snow, ice, frost or a combination of liquid water and snow/ice (also known as slush). So if it would still be snowing in the beginning of April, you’re not required to use winter tyres. Summer tyres are not forbidden during the winter, but in case of winter conditions then you’re required to use snow chains if you have summer tyres on the car. You’re never allowed to combine studded with non-studded tyres, because both types of tyres have a very different grip on the road. This could lead to unpredictable driving characteristics and therefore dangerous situations.