Is zakat obligatory on inherited ornamental gold that exceeds the niṣāb, knowing that part of it is worn and another part is kept for memory, as a gift, or for sale? And is it permissible to pay zakat in installments for past years?

Gold kept for sentimental reasons is subject to zakat if it reaches the nisab. As for gold designated for adornment, it is not subject to zakat merely by the intention of selling it. Rather, zakat becomes obligatory if the intention for it shifts from adornment to another purpose, such as buying a car or similar items, and it reaches the nisab. Zakat for past years must be paid immediately without installment, unless there is an overriding benefit or a preventing excuse, because delaying it is considered a forbidden procrastination.