It will likely rain

Rain *is infinitive in this construction.

Señor Faldaje, I'm gonna need this 'splained to me. If you get rid of the adverb, the sentence just reads "It will rain," which is just the future form of the verb. Are you saying that this is the infinitive because it takes the same form as "to rain," or is the future formed by adding "will" to the infinitive, and I've just never heard it described that way? Or some other explanation that's likely obvious but that I'm missing?