what is the poets tone in poem 18?

The tone is really affectionate and inspired. It is filled with dedication and praise. Here the theme of the ravages of time again predominates; we see it especially in line 7, where the poet speaks of the inevitable mortality of beauty: "And every fair from fair sometime declines." But the fair lord's is of another sort, for it "shall not fade" - the poet is eternalizing the fair lord's beauty in his verse, in these "eternal lines".

Check out the GradeSaver link below,

http://www.gradesaver.com/shakespeares-sonnets/study-guide/section2/