A family is one root and everything grown from it, and the reason to treat it as a unit is arithmetic: learn the endings once and a very long list of entries becomes a much shorter list of things you have to actually meet. The reason it pays less than advertised is that the endings move the stress, and a listener hears stress before anything else.
PHOtograph, phoTOgrapher, photoGRAPHic. Three members of one family with the beat in three different places, and the vowels change under it: the second syllable of photograph is a full vowel and the second syllable of photographer is not. A learner who derives the word correctly and keeps the stress where the root had it has produced something a native speaker may genuinely fail to recognise.
Eight families, and the endings that build them
Learning one word gives you four if you know what the endings do. This is the cheapest vocabulary growth available in English.
- decide, decision, decisive, decisively, verb, noun, adjective, adverb, and the stress moves twice across the set.
- analyse, analysis, analytical, and note the spelling change and the stress shift between the first two.
- succeed, success, successful, and the verb has two c and two e where the noun has two c and one.
- strong, strength, strengthen, an adjective giving a noun and then a verb.
- able, ability, enable, disabled, showing a prefix and a suffix working on the same root in opposite directions.
- -tion makes a noun from a verb, and it always pulls the stress onto the syllable before it.
- -ise makes a verb from a noun or adjective, spelled with a z in America.
- -ful and -less are opposites, giving careful and careless from one noun.
The endings that generate, and the meanings that split
Some suffixes are reliable enough to use on a word you have never seen written. Add -tion, -ment or -ity and you have a noun. Add -ise or -ify and you have a verb. Add -al, -ous or -ive and you have an adjective. That much genuinely saves you the work of learning four entries where one would do.
What it does not save you is the meaning. Two adjectives from one root regularly mean different things, and the difference is not derivable from the endings. Historic means important enough to be remembered; historical means to do with the past. A historic building is famous and a historical novel is set in another century, and swapping them produces a sentence that is grammatical and says something you did not mean.
Where it stops: the derived form that exists and means something else
This is the stall, and it is caused by the family idea working. Once a learner trusts the endings, they generate confidently, and generating confidently is right most of the time. The failures are the pairs where English kept both forms and gave them separate jobs, and those failures are invisible from the inside, because the word produced is a real English word spelled correctly.
Childish and childlike are the clearest example: one is an insult and the other is close to a compliment, and both are regularly derived from the same root by the same kind of learner. Nothing about the suffixes announces that one carries a judgement. You find out when somebody's face changes.
Where the stress is written down
Merriam-Webster prints the stress mark on every entry and gives audio for each derived form separately, free and with no account. That second part is what matters: it is easy to look up a root, hear it, and assume the family follows, and the whole point of this page is that it does not. It replaced the assumption that the beat stays where the root put it, which is the assumption everybody makes and nobody is ever told is wrong.
Look up the derived form rather than the root, every time, and listen before reading the definition. The definition is usually guessable and the stress never is.
What it looks like when it goes wrong
The audible failure is a correctly built word said with the wrong beat: PHOtographer, ecoNOMics said as ECOnomics. It is not heard as a mispronunciation. It is heard as a word the listener does not know, and they will ask you to repeat it, which is a specific and recognisable experience.
The quiet failure is the wrong family member in writing: the economical situation, a historical occasion. Both pass a spellchecker, both are real words, and both change the sentence. Nobody queries them, because a reader assumes you meant what you wrote.
The endings above are the reason a frequency list of families is much shorter than a list of words, which is what makes the vocabulary that carries essays, papers and lectures a term of work rather than a year of it, and the pairs that split in meaning have to be stored with their partners, which is which words English puts together, and which it never does. Both are under building the word count you need to say what you mean.
Since the thing going wrong is where the beat lands rather than which word you chose, the repair is not on this page at all. It is an ear problem before it is a vocabulary problem: being understood the first time you say it.