This is a strange conversation. 1. He used "would" to emphasize he wanted to know the what led her to say that. It is a subtle difference from saying "did" 2. "substantial gift" means an important gift. She is referring to her psychic ability. 3.Pete is being sarcastic. He does not really mean "Thanks". He thinks Phoebe knows very well that he doesn't want her to tell Monica that the woman in Japan is made up. Another example in English we will say "Thanks for nothing" when someone does not do what we want. Hope this helps. Let me know if you have nay questions.

Re: "Thanks" There is a common "joke" - Person A will say "forget you ever heard anything" or "promise not to say anything" or "please don't tell anything." when they want person B to keep a secret. Person B will then reply "what do you mean?" or "what secret?" or "i've forgotten already." The implication is that person B is never going to reveal the secret. They are trying to convey that they will keep so silent about the secret, they won't even talk about it with person A. They definitely won't talk about it with anyone else. So, Pete is asking Phoebe to keep quiet to Monica. (like person A). Phoebe answers in a common way "Tell her what?" (again, like person B in the "joke"). Pete thanks her, because he thinks she's really saying that she'll keep the secret. (it isn't sarcastic at all. It's sincere.) The reason people laugh at Phoebe's answer "my memory sucks" is because the audience also thinks that Phoebe is playing along with the joke, until she says that she's really actually forgotten.

Like much dialog, this dialog is full of idioms and indirect ways of saying things. You have to know a foreign language and culture quite well to understand this kind of language. "Substantial gift" is usually heard in a different context. "Substantial" by itself is a fancy way of saying "solid," "massive," "weighty," "heavy." When people talk about money it is sometimes thought to be vulgar or improper to speak too directly. The billionaire Howard Schultz has complained about being called a "billionaire" and asked to be called a "person of means." Similarly, large money gifts that a wealthy person might make to a charity would be politely called "substantial gift." If a charity says someone made a "substantial gift," we think "a million dollars, not $25." When she calls her psychic ability a "substantial gift" she's comparing it to something major, like a car or a million dollars. Where things get tricky is that she may be making a joke. She may be making a humorous overstatement. In other words, "substantial" means "huge," but even a native speaker has trouble trying to understand "how she means it."

my 4th question: is gift here a present? or a talent ?

The way it looks is a arguement. This is called context, they are basically fighting. What seems to be happening is pete is questioning why should would say something so Pretentious. Phobe says Substantial which means Great, Large, Momumental