Indeed, as the previous commenter noted, only auxiliary verbs get contracted. In fact, I believe you can think of main / auxiliary verb pairs as homographs, i.e. distinct words with different meaning and pronunciation but spelled the same. In general, main verbs are pronounced with a fully realized vowel sound, while auxiliary verbs get a weak vowel or schwa. For example,
"She said that she had had lunch already, and that that was the reason she wasn't hungry."
no subject
Indeed, as the previous commenter noted, only auxiliary verbs get contracted. In fact, I believe you can think of main / auxiliary verb pairs as homographs, i.e. distinct words with different meaning and pronunciation but spelled the same. In general, main verbs are pronounced with a fully realized vowel sound, while auxiliary verbs get a weak vowel or schwa. For example,
"She said that she had had lunch already, and that that was the reason she wasn't hungry."