3. lease: —I think this means "lease on life," mainly because that meaning seems to fit with the conclusion of the poem.

4. forfeit to a confined doom: subject to a limited duration.

5. The mortal moon hath her eclipse endured: Many scholars have seen this line as a reference to an occasion when Queen Elizabeth overcame some predicted danger, but there is wide disagreement about what the danger might have been.

6. sad augurs mock their own presage: Pessimistic astrologers now ridicule their own former predictions.

7. Incertainties now crown themselves assur'd: Desirable events, once doubtful, are now crowned with certainty.

8. endless age: i.e., without foreseen end.

9. drops: dewdrops.

10. subscribes: yields; surrenders.

11. spite of him: despite death.

12. insults: triumphs.   speechless tribes: illiterate races.

13. in this: in this poem  monument: tribute to the beloved; funeral monument.

14. crests: trophies in a tomb.   spent: wasted away by the passage of time.