1. increase: fruit, offspring, progeny.

4. tender: youthful.

5. contracted: betrothed.

6. self-substantial: of one's own substance. —I think that this line contains an implied comparison between the beloved and a candle, which burns down as it burns bright.

10. only: peerless. gaudy: bright, sparkling, like a "gaud" (jewel).

11. content: (1) what is contained within, i.e., potential fatherhood; essential beauty; contentment.

12. tender churl, mak'st waste in niggarding: —This line contains two oxymorons. A "churl" is stereotypically an old man, but "tender" means "young," so "tender churl" might be translated as "young codger." Also, a "niggard" is a hoarder, a skinflint, someone who wastes as little as possible, but this young man "mak'st waste in niggarding," wastes his beauty by keeping it all to himself.

14. To eat the world's due, by the grave and thee: to use up what you owe the world ("increase," i.e., beautiful children), by sending your beauty to the grave because of your refusal to beget children.