When to the sessions of sweet silent thought
I summon up remembrance of things past,
I sigh the lack of many a thing I sought,
And with old woes new wail my dear time's waste:
Shakespeare begins with a subordinate clause to provide an element of suspense; this draws the reader in as they await the main clause. 
 The  alliterative , sibilant “s"s in "sessions”, “sweet” and “silent” mimic the sound of sighing, — the word “sigh” appears in line three. 
 The first  quatrain  has a gentle, even, heavy rhythm — perfect  iambic pentameter  that is appropriate to the mood of the pensive speaker.
Sonnet 30