§53. The Regular Latin Diminutive Suffixes -ULUS and -CULUS
The standard rule for Latin diminutives is quite straightforward. For nouns of the first or second declension, the regular diminutive suffix is -ulus (M), -ula (F), or -ulum (N), depending upon the gender of the original noun; for words of the third, fourth, or fifth declensions, the suffix is -culus, -cula, -culum. As the suffix will suggest, the gender of the original noun is maintained in the gender of its diminutive.
Let's first consider the -ulus -ula -ulum type. If we take the Latin 1st declension feminine noun forma ("shape," "form"), we discover that its diminutive is form- (word base) + -ula (suffix) = formula ("little shape," "little form"). It was as simple as that, in Latin. From the English vantage point, it's often slightly harder, because there aren't many Latin diminutives that have come into English so totally unchanged as formula. Let's try the 2nd declension masculine noun modus ("measure," "manner"); a "little measure" is a mod-ulus, the transparent origin of the English word module. In the same fashion, Latin nodus ("knot," "node") produced the diminutive nod-ulus ("little knot"), and the English derivative nodule. To an ancient Roman, the word circus meant the great race-course in the city, the Circus Maximus; a "little circus" is a circ-ulus, only slightly disguised as English circle. For a 2nd declension neuter example we can choose granum, "a [kernel of] grain." Its predictable diminutive was gran-ulum, English granule.
What about the -culus -cula -culum suffix of the 3rd, 4th, and 5th declensions? Joined to the original word with or without the connecting vowel -i-, it presents no serious problems. If you diminish pars, part-is, F ("part"), you get part-i-cula ("little part"), English particle. From the 4th declension noun artus, M ("joint') comes the diminutive art-i-culus ("little joint"), English article. For 3rd declension neuter nouns like corpus or opus, the suffix appears added to the nominative form, producing the diminutives corpusculum (E corpuscle), "a little body," and opusculum (English opuscule), "a small or trifling work." The most startling etymology, no doubt, is mus-culus (English muscle), "a little mouse." Muscle and corpuscle are just two of perhaps a hundred or more diminutives that occur in the language of anatomy and medicine.