Hello Klox, One thing to bear in mind when you are using reduced adverb clauses, like "when reading a book", is that, theoretically at any rate, the subject of the main verb and the implied subject of the -ing form must be the same. In other words, if we take a sentence like, "When reading a book, the cat came and sat on my lap," technically you are saying, "When the cat was reading a book, it came and sat on my lap." This is unlikely to be an issue when you are speaking, as very few of us have cats that can read :) However, if you are entering English examinations such as TOEFL, it is something you should be careful about. If you want to know more about this specific aspect of your question, you could try searching Google under "dangling modifiers". Hope this helps, Alvin

actually, both statement are perfectly fine to say, although "when you are learning a language" is more straightforward in giving lets say, an instruction or command, to the one you're talking to. To me this is a bit aggressive. "when learning a language" on the other hand is quite blunt which could either referring to someone or to the one whom you're talking to. :)

Hello Klox100 You can use both: -When reading a book you have to concentrate your mind on the context. - When you are reading a book you have to concentrate your mind on the context. The difference is that by using "when reading a book" and omitting the personal pronoun "you" will make the choice of the second clause of the sentence more flexible. It could hint to the same person "you " or just mean anyone in general. - When reading a book one has to concentrate his mind on the context.

There are a few reasons, but the mainly one is: 'When reading a book' is more impersonal, it might be referring to any person. 'When you're reading a book' obviously is more personal, when meaning the one to whom it is spoken. Note that it can mean a generic 'you' too, 'you' then meaning 'anyone'.