adverb
1
BAD : He left the house at five in the morning, when the family was yet asleep.
GOOD : He left the house at five in the morning, when the family was still asleep.
BAD : I've only been here two weeks and everything is strange yet.
GOOD : I've only been here two weeks and everything is still strange.
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Yet means 'up to the moment of speaking' and is used mainly in questions and negative sentences: 'Do you feel any better yet?' 'The post office isn't open yet.'
When you want to say that an earlier state or situation has not changed, use still : 'I've taken the medicine but I still feel terrible.' 'Does Hilary still go to the same school?'
2
BAD : I didn't finish my thesis yet.
GOOD : I haven't finished my thesis yet.
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Yet (= up to the moment of speaking) is usually used with the present or present perfect tense (NOT the past tense): 'Has the taxi arrived yet?' 'Is the taxi here yet?'
3
See note at BUT (↑ but )