Paul has neither infinitive nor imperative.
Always start with the root:
Present Tense:
A vav is placed between the 2nd and 3rd letter of the root, then the suffixes are added:
sgl. m.: סגור
sgl. f.: סגורה
pl. m.:סגורים
pl. f.: סגורות
Past Tense:
The auxiliary verb להיות is needed. He was closed. הוא היה סגור
Future Tense:
The auxiliary verb להיות is needed. He will be closed. הוא יהיה סגור