solus

solus, sola, solum looks like an ordinary first-and-second declension adjective and mostly behaves like one, but it belongs to the small awkward squad that takes genitive -ius and dative -i in all three genders: solius, soli. The other members are unus, totus, nullus, ullus, alius and alter, and they are worth learning as a block rather than one at a time.

Latin uses the adjective where English reaches for an adverb. solus venit is literally "he came alone" but translates comfortably as "he was the only one who came". The neuter solum does adverb duty for "only", and non solum ... sed etiam is the standard "not only ... but also". Note that the soli in soli deo gloria is a dative singular, not a plural, which is a real ambiguity in written Latin.