When turning nouns to adjectives, what is the rule for using the suffixes -ous or -ful? Why do pain/harm became painful/harmful and not painous or harmous? Why do glory/nerve become glorious/nervous and gloryful/nerveful?
There are a variety of small differences, but I can't think of any simple ones.
The suffix -ful comes from English's native inherited Germanic vocabulary ("Germanic" here refers to a group of languages; English is not descended from the modern language called "German"), and the suffix -ous comes from Latin through French. There are a lot of words that got established with these affixes before these two sets of vocabulary mixed. For example: glorious can be traced back to Latin gloriosus; Latin did not have the -ful suffix. The adjective glorious entered the English language as an entire word around the same time as the noun glory; it was not created from the English word glory + the English suffix -ous (even though it can be analyzed that way if we ignore the history and look at it in an abstract, timeless sense). The significance of this is that when we already have a derived word for something, we're less likely to coin a new, synonymous derived word. This is called blocking. In this case, it is not absolute, but it still has had an effect.
However, there are also many words that were coined in English after these two suffixes became productive, and in these we quite happily blend Germanic root-words and French/Latin suffixes, or vice versa. Some mixed words are painful, delightful, righteous, thunderous. So in many cases, origin is not useful for determining which suffix to use, even if you can figure out the origin of the root word. I think you'll just have to memorize it; sorry I can't offer better advice!