Practice saying the following numbers.
Notice that when the number is a multiple of ten, the stress is at the beginning of the word, such as in:
Twentieth
Thirtieth
Fortieth
Fiftieth
Sixtieth
Seventieth
Eightieth
Ninetieth
Notice that when the number is not a multiple of ten, the stress falls on the second part of the word, such as in:
Twenty-eighth
Thirty-ninth
Forty-fifth
Fifty-sixth
Sixty-eighth
Seventy-third
Eighty-eighth
Ninety-fourth
Therefore, the difference between TWENTIETH and TWENTY-EIGHTH is very much about the appropriate stress pattern.
Notice what happens with numbers in the “teens.” The stress, again, falls on the second syllable of the word. Also, when you say these words, the “t” in “teen” sounds like a “t.”
thirteen
fourteen
fifteen
sixteen
seventeen
eighteen
nineteen
With multiples of ten, notice that the “t” in the words often changes to a “d” sound because “t” is surrounded by voiced sounds. Therefore, it is easier to make it voiced, as well.
twenty – sounds like “twenny”
thirty – sounds like “thirdy”
forty – sounds like “fordy”
fifty – “t” stays the same because “f” is voiceless
sixty – “t” stays the same because “ks” is voiceless
seventy – sounds like “sevendy”
eighty – sounds like “eighdy”
ninety – sounds like “ninedy”