How do I write an Assertion?

How to write assertion for d flipflop and interface

In reply to amn.rbk5793:

How to write assertion for d flipflop and interface

If you have to ask, then perhaps a good book or a search on assertion training would help you understand the concepts.
BTW, an assertion on a D FF is never done. I presume the question was more for concepts.
Ben SystemVerilog.us