1. yourself: i.e., only your essential self, not subject to time and change.

3. Against: in expectation of.  this coming end: i.e., the certain end of your life.

3-4. you should . . . your sweet semblance to some other give: you should give your beautiful appearance to another person (i.e., a child).

6. determination: end.  — Like "lease" in the previous line, "determination" is a legal term; the poet's point is that the fair youth's beautiful appearance is not something he owns, but only something for which he has a lease which will soon end.

8. When your sweet issue your sweet form should bear: i.e., when your beautiful children will reproduce your beautiful appearance.

10. husbandry: prudent management.

12. barren rage: ravaging which produces barrenness.

13. unthrifts: those who waste their possessions.