You're right, 'tis not common to write like this anymore! 'tis is a shortened form of "it is" 'twill is a shortened form of "it will" You may also come across 'twas which is a shortened form of "it was". You might recognize this from 'Twas the night before Christmas. There is also 'twixt or betwixt, both of which mean "between". Hope this helps!

The apostrophe ( ' ) is usually used to replace the letter "i". For example, instead of "it is", people used to take away the "i" and say " 'tis ". Same this with " 'twas ". It's supposed to be "it was". They abbreviated words with apostrophes like this in the same way that we currently abbreviate "can not" into "can't". Or "it is" into "it's". I hope that makes sense :) However, they used to abbreviate words like that in the olden days. (Shakespearean times I think). No one uses this anymore. If you used this in front of native English speakers, you might get a few giggles! Good luck :)

When I was a kid, in elementary school kids would "argue" in this fashion: It's mine! 'Tis not! 'Tis too! 'Tis not! 'Tis too! I guess I need to check in with my grandson and find out if it's still true today.

Yeah, Peachey! I imagined this, that there was a rhythm in these sentences!

Great answers below. :) Generally, an apostrophe means some letters are missing, so with a bit of imagination you can work out what should be there. For example, 'tis = it is. A more obscure example in Hamlet is 'sblood, which means "(by) God's blood". Interestingly, the Aussie slang exclamation "Struth!" came about exactly the same way. The main reason Shakespeare wrote like this was to keep the poetic rhythm of the sentence.