Why is "です" used here?

[Sergidh]

I found this sentence and I didn't know why "です" was put at the end of the sentence. It should mean "I want to read this book". I know the form "i-base+たい" is used, but I haven't read anything about then use the copula, could someone explain?
この本ほんを読よみたいです

 

[lanthas]

That's simply to make the sentence more polite. The -たい-form of a verb acts like an い-adjective, which also explains its negative (早い - 早くない / 読みたい - 読みたくない). With the -たい form you may use が instead of を which should make things more recognizable:

この本が読みたいです (This book is want-to-readable; similar mechanism to 好き "likeable")

You can also flip the order: 読みたい本 "the book that I want to read".

More in general, you may see verbs in the short form followed by です which again simply makes the sentence more polite. For example, while the polite form of 早かった would usually be 早いでした, 早かったです is also valid.

 

[Mike Cash]

That's simply to make the sentence more polite. The -たい-form of a verb acts like an い-adjective, which also explains its negative (早い - 早くない / 読みたい - 読みたくない). With the -たい form you may use が instead of を which should make things more recognizable:

この本が読みたいです (This book is want-to-readable; similar mechanism to 好き "likeable")

好き is formed from 好く.

You can also flip the order: 読みたい本 "the book that I want to read".

Which also changes the meaning/nuance, so if you're going to introduce that, you might want to flesh out your explanation.

More in general, you may see verbs in the short form followed by です which again simply makes the sentence more polite. For example, while the polite form of 早かった would usually be 早いでした, 早かったです is also valid.

Have you actually encountered something like 早いでした before? I never have. Other than as a mistake commonly made by foreign learners of Japanese, I mean.

 

[Toritoribe]

です in 読みたいです is a suffix which is used just to add politeness to the i-adjective, not a copula. This です never conjugates, as Mike-san wrote.

 

[lanthas]

With "similar mechanism to 好き", I merely wanted to point out that what would be the object in English in sentences like "I want to read this book"/"I like this book", is the subject in Japanese ("本が読みたい"/"本が好きだ"). Or is this incorrect as well - that is, does が have other functions aside from marking the subject (like を has other functions than marking the direct object)?

Looking up 早いでした on Google does turn up quite a lot of results - on websites that are clearly not created by foreigners - but now that I look more closely, they are mostly blog posts and comments where loose grammar can be expected. And looking back at my textbooks, even Genki I shows that です does not conjugate after い-adjectives... Sorry for the mistake. I guess the original question is fully answered now.

 

[Mike Cash]

Looking up 早いでした on Google does turn up quite a lot of results - on websites that are clearly not created by foreigners - but now that I look more closely, they are mostly blog posts and comments where loose grammar can be expected.

Your post inspired me to do a Google search as well. Searching without enclosing quotation marks yields tons of results...with 早い and でした generally being in separate parts of the text and not adjacent each other. Searching with enclosing quotation marks yielded far fewer results. Most show some intervening punctuation which indicate the でした wasn't there to conjugate the 早い. One of the top results not fitting that description was from a wiki page and is (I believe) simply an error probably put there by a foreigner. Another of the top results was a blog that I suspect was not written by a native speaker. At any rate, results have to be individually sampled to see what you're getting when you check Google for existence or prevalence of any Japanese word/term/phrase because of the sort of things mentioned above.

 

[nekojita]

Be careful of using google hit numbers, as well - often initially they list a large number which is an estimate. If you go forward enough that drops (in this case there I get only ~200 real hits). As Mike says, a lot of these are not actually 早いでした but stuff like 「メールの返信が早い」でした。

For comparison, there are no hits at all in the 少納言 corpus (covers a wide range of written Japanese from textbooks and law to Yahoo blogs and 知恵袋).
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