Structure
What are the elements that make up this word's structure?
I believe that congenial is composed of:
con + gene/ + i + al
I know consent and condition.
The suffixal construction <-ial> is composed of a connecting vowel letter and the suffix <-al> that we see on words like final and removal. A suffixal construction is a common occurrence of either two suffixes side-by-side or a connecting vowel letter and a suffix. We say it's suffixal, because it's not a suffix of its own, but it behaves like a suffix because it comes after the base. The <-al> suffix on suffixal creates an adjective form for the stem
Before we check for evidence, let's look at congenital.
con + gene/ + it + al
If this is actually the word sum, then that would mean it shares a base with congenial. However, we must check out the evidence in Etymonline first.
The entry for congenial tells us this word only came to us in the 1600s, and its current meaning came about in the early 1700s. At first it meant "partaking of the same nature." Then we see "assimiliated form" of Latin com * + genialis. Even though this looks like a word sum, it is not. Doug Harper, the author of Etymonline, provides it as a record of the history of the word's structure. It does however provide evidence that Assimilated prefixes had changes to the phonology that are reflected in the orthography. The final grapheme of the prefix often matches the first grapheme of the base or is easier to pronounce. In this instance, an
This assimilation occurred in Latin, so it is not something that would be reflected in a word sum, like a "replaced synchronic processes, not diachronic. Synchronic means "at a point in time;" diachronic means "through time."
For the second part, genialis, we have to know how to take a Latin word and find the English base. The <-is> is a Latinate suffix we can remove, as is the <-al> Latinate suffix (which we also have in English). We are left with
con + geni + al
But that isn't all. Notice the last sentence in the entry says to compare congenital. Any time Etymonline says to see another entry, there's more to the story.
In the entry for congenital, we see this word came to us even later, 1796. You may notice in Etymonline that as we go back in time, dates of entry are more general, by century or "circa." Later, we can be more precise with our dating.
This says that part of congenital comes from genitus, the "past participle" of gignere, "beget." Let's talk about past participles for a moment. We have them in English as well. A past participle will often look like a past tense verb. We can have a "smoked ham" for dinner. The past participle looks similar to the past tense of smoke. We can use past participles as part of a verb phrase:
We have smoked a ham every Christmas for the past ten years.
We can also use past participles to modify nouns like adjectives do.
We enjoy having smoked ham sandwiches the day after Christmas.
Going back to the entry for congenital in Etymonline, we learn that the sense of "existing from birth" once belonged to congenial. However, in the ensuing years, congenial has taken on more of a sense of "agreeable."
When we look at genitus, we can remove the Latin suffix <-us> to arrive at
con + genit + al
The genitus is what is known as a stem suffix. Stem suffixes may also be removed on occasion or possibly analyzed.
con + gen + it + al
I'd offer the base in this word is
con + gene + it + al
So is it possible that's what's happening with congenial as well?
con + gene + i + al
This goes back to the being a connecting vowel letter instead of part of the base
If we analyze this far, these two words, ultimately related to Latin gignere have many relatives.