see AskOxford: 'tis
• contraction chiefly literary it is.
'Tis indeed archaic.
'Tis common in Shakespearean langage.
Wouldn't use it in modern speech.
"Tis sorry I am to hear it." It is sorry that I am, to hear it (I'm sorry to hear it).
"Tis, thanking you I am" It is thanking you I am (I am, indeed, thanking you).
Hope this helps