From 11:00PM PDT on Friday, July 1 until 5:00AM PDT on Saturday, July 2, the Shmoop engineering elves will be making tweaks and improvements to the site. That means Shmoop will be unavailable for use during that time. Thanks for your patience!
Apostrophe is when the speaker talks directly to someone who isn’t there or something that can’t actually be talked to, such as the Tyger. The whole poem is addressed to the Tyger. Can the Tyger talk? No. Does it even exist in a concrete sense? Probably not. The apostrophe helps the poet keep the subject alive and in-your-face, rather than talking about a bunch of generalities.