Explanation:
Some words are made up of a root and a suffix:
dangerous = root —> danger + [b]ous[/b] —< suffix
commercial = root —> commerc(e) + [b]al[/b] —< suffix
In some words with suffixes, the stress stays on the same syllable as in the root. Compare:
‘danger and: ‘dangerous
In other words, the suffix changes the stressed syllable. Compare:
‘commerce and: com‘mercial
Suffixes which don’t usually change the stress pattern in the root word include (however they maybe some exceptions.) -able, -age, -al, -er, -ful, -less, -ness, -ous and -fy. For example:
comfort — ‘comfortable
per‘cent — per’centage
e‘lectric — e‘lectrical‘amplify —‘amplifier
re‘gret — re‘gretful
re‘gard — re‘gardless‘foolish — ‘foolishness
di‘saster — di‘sastrous‘beauty — ‘beautify
(However they maybe some exceptions.) Exceptions with -able and -al include:
ad‘mire — ‘admirable
pre‘fer — ‘preferable‘medicine — me‘dicinal‘agriculture — agri‘cultural