今天的《词汇大师》讨论的是动名词和不定式，在许多主谓句中动名词和不定式都可以使用，我们只要记住不定式 to+verb 语义比较正式，对比一下：Swinming is a good exercise. To swim is a good exercise……

AA: I'm Avi Arditti with Rosanne Skirble, and this week on Wordmaster: to be or not to be, or should there be an -ing? That is the question as we look at gerunds and infinitives.

RS: To be, to run, to eat: the "to" indicates the infinitive form of the verb. But if you were to use these verbs as gerunds, they would take an ing as in being, running and eating.

AA: Juan, a listener in Chile, is not always sure when to use an infinitive and when to use a gerund. He sent us four sentences and asks if they're right:

RS: Here they are: To swim is a good exercise. To work ten years in the mine is enough. Sleeping is a luxury. Being able to read is important.

AA: A good question for English teacher Lida Baker in Los Angeles:

LIDA BAKER: "The very simple answer to Juan's question is: yes, that the gerund and infinitive are more or less interchangeable when they are in subject position. Which is the way the sentences that he offered as examples — notice that the gerund and infinitive are at the beginning of the sentence, they are the subject. And I think we should probably, for those who might not remember, just point out that a gerund is the -ing form of a verb but used as a noun. So, 'swimming is good exercise.'"

AA: "But the way he says here, 'to swim is a good exercise,' now to my ear it didn't sound like native English to say, 'To swim is a good exercise.' I probably would have said 'swimming is a good exercise,' 'working ten years in the mine is enough.' Not 'to work ten years,' 'to swim is a good exercise.'"

RS: "It's a little bit more formal to use the 'to' plus the verb, the infinitive."

AA: "Yeah."

LIDA BAKER: "But what about the sentence, 'To be able to read is important'? To my ear, that's absolutely correct."

AA: "I agree with you."

LIDA BAKER: "When we talk about infinitives and gerunds in subject position, at the beginning of a sentence, a person who's learning English probably needs to know that the meaning is more or less the same."

RS: "What's the difference in other positions between the gerund or the infinitive."

LIDA BAKER: "Gerunds or infinitives can occur in all the positions that nouns normally occur. So we've already seen Juan's example in the subject position. They can also occur after the 'be' verb. So you have a sentence like, 'Her dream is to become an opera singer.' Or, 'My hobby is playing the piano.' They can occur in what's called an appositive. An appositive is a noun that comes after another noun where the second nouns explains the first noun. So in a sentence like 'I appreciate your offer to take me to the airport,' we have the noun 'offer.' And what is the offer?"

RS: "To take me to the airport."

LIDA BAKER: "Right, so there are two noun structures there, one of which defines the other one. So that's a way that we use infinitives and gerunds. The appositive structure, we always use an infinitive. So, 'I appreciate your offer to take me to the airport.' We would never say, 'I appreciate your offer taking me to the airport.' Right?"

AA: "Right."

LIDA BAKER: "That's absolutely wrong."

AA: "Well, you said — wait a second, you said an appositive is a noun that follows another noun."

LIDA BAKER: "Uh-huh."

AA: "So 'to take' in that case, even though it's an infinitive form of a verb, it's being used as a noun?"

LIDA BAKER: "Yeah, infinitives and gerunds are almost always used as nouns. That's part of the definition."

AA: "OK, I knew that as a gerund, but I didn't realize that was true with an infinitive."

LIDA BAKER: "Not in every case, but in almost every case. Do you want to hear a couple of other?"

AA: "Please!"

LIDA BAKER: "Object of the preposition, and these are always gerunds. So, 'Thanks for helping me.' The preposition 'for,' and the word 'helping' is the object of the preposition. In that case, we always use a gerund.

"And finally, the one I want to mention is the infinitive or gerund used as a direct object. That is to say, the object of a verb. And the reason I mention this last is that this is the one that is the biggest challenge for people who are learning English. So are you ready?"

RS: Not quite. We're short on time, so we'll finish the discussion with English teacher Lida Baker next week.

AA: But we will tell you about a free Grammar and Writing Guide on a Web site she found. It's sponsored by the Capital Community College Foundation, a non-profit organization in Connecticut.

RS: It's a long address, so to make it easy to find we'll post a link on our site, voanews.com/wordmaster. And our e-mail address is word@voanews.com. With Avi Arditti, I'm Rosanne Skirble.