Answer: You have to attend a seminar.
Explanation:
'Have to' is used to express general obligations, duties, or necessities (as opposed to 'must', which expresses specific obligations) in the present, the future, or the past. In the future or the past, 'must' and 'need' are always replaced by 'have to'.
Yet, in the negative form, 'don't have to' means there´s no obligation or necessity, but it can be done. ('You don't have to cook' means you can still do it if you fancy to).
