I knew that:

 - ***yet*** should be used in negative sentences and questions.
 - ***still*** should be used in affirmative sentences.
 - We can use ***yet*** in affirmative sentences containing this pattern:***have yet to***

However, I was reading a passage and came across with this sentence:

>*Yet if we look into the beliefs, we find that the rose is much more than the mere symbol of romantic love invoked by every minor poet and painter.*

1. Is it grammatical to use "yet" or we should use "still"? 

2. What would be the difference if one put "still" there?