Normally, when reading a text, people use a strategy that I call "reading for content". The goal of this strategy is to get the main idea of the text as quickly as possible and with as little effort as possible. To accomplish this goal, your brain will try to read as few words as possible and spend only a fraction of a second on each word.
For example, when reading the following passage, you don't really see it like this:
Once when I was six years old I saw a magnificent picture in a book, called True Stories from Nature, about the primeval forest. It was a picture of a boa constrictor in the act of swallowing an animal. Here is a copy of the drawing. In the book it said: "Boa constrictors swallow their prey whole, without chewing it. After that they are not able to move, and they sleep through the six months that they need for digestion."
I pondered deeply, then, over the adventures of the jungle. And after some work with a colored pencil I succeeded in making my first drawing.
To your brain, it looks more or less like this:
Once when I was six years old I saw a magnificent picture in a book, called True Stories from Nature, about the primeval forest. It was a picture of a boa constrictor in the act of swallowing an animal. Here is a copy of the drawing. In the book it said: "Boa constrictors swallow their prey whole, without chewing it. After that they are not able to move, and they sleep through the six months that they need for digestion."
I pondered deeply, then, over the adventures of the jungle. And after some work with a colored pencil I succeeded in making my first drawing.
Here are some characteristics of "reading for content":
An extreme example of "word blindness" is the rather well-known puzzle where you're asked to count how many times the letter F occurs in the following passage:
Reading for content is a great, time-saving way to extract information from printed sources. The problem is that you may not need the grammar words to understand a text, but you do need them to produce a text. So if you don't pay attention to things like articles and prepositions, you won't be able to use them correctly in your own sentences.
For example, here is a sentence from the opening paragraph of this article. Most learners (except those who are proficient in English grammar or extremely observant) will probably find it difficult to fill in the blanks:
The above explains why some learners can read a 300-page book and still have problems with relatively basic grammar. It also explains why articles and prepositions are among the hardest aspects of English to learn. The conclusion for the English learner is that if you want to improve your production (output) skills, you will have to train yourself to notice grammar words.
Here's an illuminating passage posted by Maya l'abeille at the Antimoon Forum:
I believe that seeing correct and typical English sentences helps a lot to learn how to use English properly. It is also important to read and read again every structure that is new to you, so that you can remember them. If you only read the book without taking any pause to think carefully about the "new" sentences, you will hardly remember any of them.
I've read all Harry Potter books straight myself, and when I opened them again, I realised I had viewed loads and loads of useful structures whithout remembering them - which was such a shame! I'm reading The Full Monty (Penguin Readers collection) using the "pause and think" method at present. Now after a few days of daily reading, when I take a look at an English text, many structures are familiar to me - "hey, I remember reading this one in The Full Monty!".
Therefore, I believe this method is efficient and I would advise it to all learners.
Sometimes, we don't realise how wealthy a single book can be - loads to learn just in one of them.
I agree with Maya l'abeille about the "pause and think" method. Here's the process that I recommend for dealing with sentences in texts:
If you don't like to stop reading (to look up a word in your dictionary or add a phrase to SuperMemo), you can write down all the interesting sentences, or you can underline them in the book with a pencil. This way, you can handle these sentences later.
Another important piece of advice is that you don't have to use the above strategy all the time. Reading in this mode can be quite exhausting, so don't do it when you're tired after a long reading session. Also, do not try to give equal attention to every sentence. Some sentences in books (e.g. long poetic descriptions) do not contain phrases or structures that are useful for building your own sentences. Some characters in books use weird slang expressions which aren't very useful either.
Finally, the "pause and think" technique will not always make you remember the exact way to say something. But perhaps you'll remember that this particular type of sentence is "weird" or "difficult" in English. If you remember that, it will at least make you stop before you write that sentence, and look it up instead of making a careless mistake.
I'll now give you a short demonstration of the "pause and think" method. Here are two English sentences and examples of thoughts that you should get when reading them:
If you think you don't have time to read, try to carry a book with you everywhere you go. That way, you can read when you're waiting in line, waiting for a bus, or even when walking (but make sure you don't walk into other people or vehicles).