Stressed syllables in "ostentatious" and "adventurous" [duplicate]

Solution 1:

There are a few ways to explain their stress patterns.

One way is to say that it is based on the stress patterns of related nouns, as you said: ostentation and adventure.

Another way is to look at the exact form of the ending of the word.

Ostentatious ends in -ious. Words ending in -ious are pretty much always stressed on the immediately preceding syllable (I don't know of any exception). (This rule applies regardless of whether -ious is pronounced as one or two syllables.) Incidentally, the same stress pattern applies to words ending -ial, -ian or -iate, as well as -eous, -eal, -ean, -eate.

Adventurous ends in -ous preceded by a single consonant letter. Words of more than two syllables that end like that can be stressed on the syllable immediately before -ous (as in intravenous or heterozygous), but are more often stressed on the preceding (third to last) syllable: carnivorous, humorous, ominous, frivolous.

While it's true that the stress pattern usually corresponds to the weight of the second-to-last syllable, it's a little unclear whether this explains the stress pattern since you could make an argument that stressed vowels are lengthened in this context (rather than long vowels being stressed).