grammar - Questions about comparatives

I have two questions that I think are quite related regarding the interpretation of comparatives.

1. Is it necessary to include a より or a 方 expression to convey a comparative?

Suppose you are having a conversation about how the book you just read was too long. You are then asked what you want to do later. You respond:

昨日読んだ本より短いのが買いたい。

I want to buy a shorter book than the one I read yesterday.
(Please bear in mind that I constructed the Japanese sentence above, so it could be wrong in any number of ways. I'd appreciate a correction if required.)

My question is, if you responded with simply:

短い本が買いたい。

would it be interpreted as implying "shorter" (where "shorter" is drawing a comparison to the book previously mentioned in context), or "short" (where "short" is not explicitly compared to anything), or perhaps it is unclear?

Furthermore, is there an alternate way to express the comparison without explicitly stating 「昨日読んだ本」?

2. Do なる expressions imply comparison or must it be explicitly stated?

I'd like to compare the following English sentences:

1. 
   

   The road became wide.

   
   


2. 
   

   The road became wider.

   
   

Sentence 1 states that something changed and now the road is wide (as opposed to narrow). Sentence 2 states that something changed and now the road is wider than it used to be. It does not actually state that the road is wide.

As far as I know, both of these sentences would be translated to:

道が広くなった

Is my translation correct for both cases and if so, how could it be altered to differentiate between the two English meanings?

Answer

"Is it necessary to include a より or a 方{ほう} expression to convey a comparative?"

In many cases, yes, but certainly not in all cases.

The key factor here is whether or not the comparison is made against a concrete example as the standard. That is to say, one needs to consider whether the comparison is absolute or relative. In other words, is it objective or subjective?

「昨日読{きのうよ}んだ本{ほん}より短{みじか}いのが買{か}いたい。」

In this sentence, the comparison being made is basically objective. Why so? Because the speaker knows exactly how long the book he read yesterday was. Today, he wants to buy a shorter one.

(For those wondering, the use of 「が」 there is completely natural.)

「短い本が買いたい。」 ("I want to buy a short book.")

This sentence, when uttered all by itself, can only mean "I want to buy a short book." It does not mean "I want to buy a shorter book."

There is, however, an easy way to express "a shorter book" without comparing it to another actually existing book. In other words, the book only needs to be "kind of short" by the speaker's own standards. That phrase would be:

「短めの本が買いたい。」 ("I want to buy a shorter book.")

How short is 短め, only the speaker knows because that is a subjective judgement. Important thing is that the sentence makes perfect sense to the speaker while it might sound ambiguous to some others.

"Do なる expressions imply comparison or must it be explicitly stated?"

Great question. They do generally imply comparison. Otherwise, what is the use of our magic verb 「なる」, which is used to express "change of state"? You cannot talk about "change" without making some kind of comparison, can you?

「道{みち}が広{ひろ}くなった。」

undoubtedly means "The road became wider (than it was at an unmentioned time)." You simply cannot say that sentence if the road width has not changed. Using 「なる」 means there has been a change.

You could say:

「以前{いぜん}より道が広くなった。」

to state the standard of comparison by adding "than before", but the point of the matter is that "than before" is already implied in the sentence without 「以前より」.