‘To’ before a verb

IT IS a grammatical rule that a verb preceded by “to” should be followed by the base word of the verb. I have come across verbs preceded by “to” that use other forms of the base word, for example: “Plying various trades refers to working in different occupations.”

Is it correct to say that “to” can be followed by a participle or perfect tense? How do I explain this to students of English language? – Rajah