'th'
There are two ways of saying the 'th' digraph. One is 'unvoiced', such as the way it sounds at the end of 'bath', 'moth' or at the start of 'thin'. One is 'voiced' such as in the words 'this' and 'that'.
There is a different action for both! Learning is below for both pronunciations.
'Unvoiced' th
'Voiced' th