The problem is that yet can be used as an adverb as well as a co-ordinating conjunction. Let’s look at its function as a conjunction first of all.

yet as conjunction

You are right, Viji. Yet is similar in meaning to but. But is a
co-ordinating conjunction used to contrast two statements:

• They can speak Arabic but they can’t read or write it.
• He tried to book a holiday on Bali, but he didn’t have enough money to pay for it.

We use yet as the preferred alternative to but when we want to emphasise that contrast to achieve a stronger effect:

• She can play the piano very well, yet she can’t read music at all.
• The yachtsman had lost all sense of direction, yet he refused to give up in his attempt to cross the Atlantic.

We sometimes put and in front of yet when it is used in this way or use even so as an alternative to yet or and yet:

• She can play the piano very well, and yet she can’t read music at all.
• The yachtsman had lost all sense of direction. Even so, he refused to give up in his attempt to cross the Atlantic.

However and nevertheless are sometimes used as more formal alternatives to yet:

• He had no chance of winning the race or even of coming in the first six. However, he kept going and crossed the finishing line ahead of his team mates.
• He had not slept for three nights. Nevertheless, he insisted on going into work the following day.

In colloquial spoken English, mind you, but still or still are sometimes used as less formal alternatives to yet:

• The weather was lousy. It rained every day. Still, we managed to enjoy ourselves.
• I don’t like the work very much. Mind you, the people I work with are very nice.
• You can be very annoying at times, but we still love you.
  

yet as adverb

When yet is used as an adverb, it is used to talk about something over a period of time, up till now:

• Is lunch ready yet?
• Are the Hunts back from their holiday yet?

It is often used with the negative when you are saying that up to the present time something has not happened. It is normally used with present and perfect tenses, though in American English you will sometimes hear it used with the past tense. Still can sometimes be used as an alternative to yet. When we use still in this way, it is emphatic. We are saying that we are very surprised that it hasn’t happened. Compare the following:

• Don’t eat the plums. They’re not ripe yet. / They’re still not ripe.
• I haven’t been to Wales or Scotland yet, though I’ve visited England many times.
• I still haven’t been to Wales or Scotland, even though I’ve visited England many times.
• Did you phone him yet? No, sorry. I forgot.

As we can see from the above examples, yet is normally used with negative sentences and in questions, but it is sometimes used in affirmative sentences in a more formal style:

• I have yet to meet the man I wish to marry.
• We have yet to learn whether there will be any survivors from the earthquake.