Here's why:
* Infinitives: These are verb forms that usually start with "to" (e.g., "to run," "to sing"). Infinitives act as nouns, adjectives, or adverbs.
* Pronouns: These are words that replace nouns (e.g., "I," "you," "he," "she," "it," "we," "they," "this," "that").
What might be confusing you?
Perhaps you are thinking of infinitive clauses, which can function like nouns, but these clauses are not pronouns. An infinitive clause contains an infinitive and other words that complete its meaning (e.g., "To be happy is important").
Let me know if you have another grammatical term you'd like clarification on!