So my entry for -ic, for example, reads as follows:
-ic stress-imposing ɪk —periodic ˌpɪər i ˈɒd ɪk || ˌpɪr i ˈɑːd ɪk
The example(s) were meant to be carefully chosen to show the stress effect (if any) of the affix. Here, -ic causes stress (and therefore a strong vowel) on the preceding syllable -od-, which in the base form period is unstressed and weak.
I didn’t attempt to provide lists of exceptions (if any). In the case of -ic there are hundreds and hundreds of regular cases, alongside a handful of exceptions such as Arabic, catholic, heretic, rhetoric.
Last week I was brought up short by the word infantilism. This word is stressed on the -fant-: ɪnˈfæntɪlɪzəm. Why?
Comparison with the base forms infant, infantile, both with main stress on the initial syllable ˈɪn-, shows that this change of word stress placement must be attributed to the suffix -ism. (I shall ignore the question of whether or not we wish to recognize a secondary stress on the suffix itself.)
Yet -ism is one of the suffixes usually regarded as having no effect on stress. Think of radicalism, clericalism, feudalism, parochialism, imperialism, capitalism, territorialism, nationalism, liberalism, journalism, naturalism, fundamentalism, creationism, magnetism, relativism, negativism and large numbers of other cases.
Are there any other words like infantilism, in which the suffix appears to impose stress on a syllable different from that stressed in the base form?
In mechanism ˈmekənɪzəm and evangelism ɪˈvændʒəlɪzəm the stress goes on the second syllable back from the suffix: compare mechanic(al) mɪˈkænɪk(əl) and evangelic(al) ˌiːvænˈdʒelɪk(əl), in which -ic has its usual effect. There’s no base form *mechan, though there is a combining form mechano- ˌmekənəʊ-. There’s a rare word evangel ɪˈvændʒəl (OED: “now arch. or rhetorical”). There’s no obvious base form for monotheism, polytheism, synergism or syncretism.
In the case of catholicism kəˈθɒlɪsɪzəm the suffix -ic regains its usual ability to impose stress on the preceding syllable, despite its failure in catholic.
There’s some uncertainty or variability about word stress placement in opportunism, obscurantism, mercantilism and Adventism.
So perhaps the best pedagogical rule would be:
-ism has no effect on word stress. There are two important exceptions: catholicism and infantilism.