Tax Year Prior to 2020: Are there instructions somewhere for how to fill out the it2104 form? Are there definitions for lines 6 - 20? Ex: what does New York State household credit (line 8) mean?

Lines 6 - 20 should be filled out based upon how your state income tax return will be completed.  If you expect to receive the credits listed on any of those line items, then you should complete those lines on IT-2014.  See more information on the 2019 IT-2014 form here:  2019 IT-2014.

The description of the lines on IT-2014 should correspond to credits available on the New York income tax return.  For more information about these credits you can review the information in the 2018 Form 201 instructions booklet beginning on page 29.  Click here to be brought to a PDF of those instructions.

Also, for more information on line 8, the New York State Household Credit, see this link:  New York Household Credit