1. How often do you read the newspaper?

2. How often do you read a newspaper?
3. How often do you read newspapers?

(What is the difference among them? Does #1 mean "How often do you read the same newspaper?")

4. That's too expensive!
5. That's too much!

(Both are the same, right? Then what is part of the speech of 'much'? Is 'much' an adjective? Does 'much' mean expensive?)

#1 means how often you read a specific newspaper.

#2 means how often do you read any newspaper.

#3 means how often do you read newspapers in general.

#4 and #5 sound the same to me.

1 - 3 mean slightly different things:

1 = "the newspaper" is most often referring to one newspaper that both the speaker and listener are familiar with.
2 = "a newspaper" means any newspaper, no matter which one.
3 = "newspapers" means about the same as #2, but in the plural.

Yes, 4 and 5 mean the same thing. In that sentence "much" is serving as an adjective in a shortened sentence; the entire expression would be, "That's too much money!"
http://dictionary.reference.com/browse/much
As you can see, it can be a noun, an adverb, an adjective -- and is used in several idioms.

The use of "the" indicates a specific thing, typically the only one. The use of "a" indicates one of several options. For example, "this is the way to do it" in contrast to "this is a way to do it." In this case, this is "the" way to use these two articles.

With 4 and 5, expensive specifically refers to cost. However, the meaning of "much" is determined by the previous context. For example, if you were being portioned a dessert at a meal, you could also say, "That's too much!"

I hope this helps a little more. Thanks for asking.

1. "How often do you read the newspaper?" - This question is asking about a specific newspaper. It could refer to a particular newspaper that you regularly read.

2. "How often do you read a newspaper?" - This question is more general and asks about your reading habits with regards to newspapers in general. It does not imply a specific newspaper.

3. "How often do you read newspapers?" - This question is similar to the previous one, but it emphasizes multiple newspapers. It asks about your habits of reading any newspapers, not just one in particular.

Regarding the difference between #1 and the other two questions, #1 does not necessarily mean "How often do you read the same newspaper?" It can refer to reading any newspaper but does not specify whether it is always the same one.

4. "That's too expensive!" and 5. "That's too much!" are similar in meaning but not exactly the same. "Expensive" refers to the cost or price of an item, while "much" refers to the quantity or amount of something. "Much" is an adverb in this context, modifying the adjective "too." It does not mean expensive but rather implies that the amount or quantity of something is excessive or beyond what is desirable.