"Since you have had a hat."
The English translation makes no sense and is not even a complete sentence. It appears that "since" in this case is referring to a cause, but that should be stated with "Parce que." If it is referring to time, it should use a tense other than the present. The pluperfect (le plus-que-parfait) would make the most sense but we're not at all ready for that tense yet.