How can a sunburn cause skin cancer?

Ultraviolet: Some wavelengths produce a trademark mutation dna. The accumulation of such mutations in a line of cells causes precancer and the common skin cancers (basal cell, squamous cell, melanoma). Sunburn is the body's attempt to rid itself of damaged cells but this does not work perfectly. A good sunscreen will let you enjoy the sun and be much safer.

Yes: Exposure to uv radiation from the sun is the main cause of skin cancer.

DNA damage: Sun burn does not cuase skin cancer. It is jsut a marker for skin damages caused by uv radiation from the sun. Uv radaition causes damage to the dna of skin cells, which is the underlying cause of skin cancer.