Help. Which one is the appropriate meaning of infinitive marker "to" in the sentence? The sentence: It's plain to see I know it means very clear, but don't know the meaning of infinitive marker "to". ■expressing purpose or intention. I set out to buy food. ■expressing an outcome, result, or consequence. he managed to escape. ■expressing a cause. I'm sorry to hear that. ■indicating a desired or advisable action. we asked her to explain.The sentence: It's plain to see I know the sentence means it's very clear.