Some words are made up of a root  and a suffix:

dangerous   =  root –> danger  +  ous –<  suffix
commercial =  root –> commerc(e)  + al –<  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