Yet - position in sentence

I would say that your grammar book, Cambridge Dictionaries and the British Library are all correct, Maria-Leena.

Yet is normally placed at the end of the clause, particularly in informal English and in questions, but can go immediately after not in negative sentences in a more formal style, such as Cambridge Dictionaries and the British Library have used. Compare also the following:

• How long have you been in Britain?
  ~ For over a year now.
  ~ Have you been to Wales or Scotland yet?
  ~ No, not yet. I haven't even ventured out of London yet.

Although she has been in Britain for more than a year, Maria has not yet visited either Wales or Scotland.

Yet - meaning and use

We use yet in questions to ask whether something has happened up to the present time. Not yet then indicates that it hasn't happened yet:

• Is dinner ready yet? I'm starving.
  ~ No, it's not ready yet. It'll be another half an hour.

In a more formal style it is possible to use yet in affirmative sentences:

• We have yet to discover whether there are any survivors from the plane crash.
  
  
• I have yet to speak to the personnel manager to discuss my future.

In a less formal style, we might say:

• We still don't know whether there are survivors from the plane crash.
  
  
• I haven't spoken to the manager yet, so don't know what my future will be.
  
  
• I still haven't spoken to the manager, so don't know what my future will be.

Thus, in negative sentences, as we can see from these examples, there is considerable overlap in meaning and use between yet and still. Still is the more emphatic of the two.

still - meaning and use

We use still in questions, affirmative and negative sentences to indicate that something is not finished and that we are perhaps surprised or concerned about this. Because it is emphatic, it often carries considerable word stress:

• Is it still raining?
  ~ Yes, it's still raining. No chance of playing tennis today, I'm afraid.
  
  
• I still don't know whether Brendan will be coming to the engagement party. I've tried to reach him several times on the phone, but can't seem to get hold of him.

already - meaning and use

Whereas still and yet normally refer to present and future circumstances, already normally refers to something that is in the present or recent past. It is mainly used in questions and affirmative sentences and usually expresses surprise that something has happened sooner than expected.

• When do you expect Polly to arrive?
  ~ She's already here! Haven't you seen her?
  
  
• Can you give me a hand with the layout for this article.
  ~ No, I'm sorry, I'm already late. I have to leave right now.
  
  
• Can you help me move those boxed upstairs?
  ~ I've already moved them.
  
  
• Have you finished that typing already?
  Yes, I finished it about five minutes ago.
  
  
• By the age of three, Mozart had already learnt to play the piano.

still / already - position in sentence

Note from the above examples that in contrast to yet, still and already usually occupy mid position in the clause.

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