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