1) The price of cell phone is becoming more and more expensive because of various features.
The prices of cell phones are becoming more and more expensive because of various new features.
There's more than one cell phone so I changed it to plural by adding an s. Cell phones have more than one price, because different models cost different amounts, so I also changed that to be plural. I changed "is" to "are" because "are" is what should be used when referring to something that is plural.
I also added the word "new" because it would more clearly explain why the "various features" are changing the prices. The prices are increasing because these features are new.
2) The Internet has made us talk more and more frequently.
As the Internet prevailed, we came to write more and more messages to each other.
These are both correct as far as I know. In the second sentence the comma may not be needed.
3) I am a big fan of Ichiro all the more because he is hard-working.
The hyphen is not needed. It should be "hard working" because in this case the description is after the noun. If you said "I'm a big fan of hard-working baseball players like Ichiro" then that would be a correct use of the hyphen because the noun is after the description.
4) He cannot speak French, much/even/still less write a letter in French.
5) I cannot look at his face, much/even/still less speak to him.
Either much or even could be used, but I think saying "much less" is more common and sounds more natural. Saying "still less" doesn't sound right to me in this situation.
6) My elder sister is no taller than me.
This is correct, but as mentioned before we normally say "older." The word "older" is more flexible because it can be used in more situations than the word "elder," and it sounds less formal. I think that in cases where either word can be correctly used it would be preferable to say "older."
7) Leaning how to spend money is no less important than learning how to save money.
There's a typo in the first word. It should be "Learning."
8) This problem is no more easy than that one.
Either "This problem is not easier than that one." or "This problem is no easier than that one." would sound okay to me, but I think "no easier" sounds more natural than "not easier." I don't know if one of those is more correct than the other.
Because the word "easy" is an adjective with one syllable and is being used in a comparative form it's correct to write it as "easier" instead of "more easy." The words "more" and "most" are normally only used if the adjective has three or more syllables and sometimes if it has two syllables (for example, "more correct" even though "correct" has two syllables).
9) I have not more than $50 today.
I have only $50 today.
I think saying "I only have $50 today" sounds the most natural. It would be possible to say "I have, at the most, $50 today" (I used commas to show a pause when speaking) or "I have $50 at the most today" if you are unsure of how much you have but you think that it can't be more than $50.
10) There are not less than 400,000 books in the city library.
With the phrases "not less" and "no less" there may be a small difference, but it's hard to explain and I'm not completely sure. I can at least say that "no less" sounds correct and natural to me. Saying "no less than" seems to imply surprise or amazement, while "not less than" seems more analytical.
11) This river is widest here.
Looks okay, but it may depend on when it is being used. If I had a map and was looking at multiple rivers I might say "this river," but if I was only looking at one river and was not comparing it to any others I would say "the river." I might be wrong about that.
12) The car can travel at 150 kilometers per hour at best.
Looks okay.
13a) The higher you climb, the colder you feel.
13b) The higher you climb, the colder you will feel.
They both look okay, but I think English sounds more natural when using contractions. Even though it's correct to say "you will feel" I think most native speakers would say "you'll feel" in most situations. I remember an old tv show that had a character who was suppose to be from China. One of the ways the script writers tried to make him sound more foreign was to make him never use contractions. That doesn't mean native English speakers always use contractions though. Not using them sounds more formal.
14) I feel happiest when I'm playing tennis.
Looks okay, but I would normally say "feel the happiest" instead of "feel happiest."
15) I like him none the less for his failure.
Looks okay, but it sounds a little unusual to me. I think it's correct.
], so that you can see:
