可是:(连词conj.)表示转折,前面常常有“虽然”之类表示让步的连词呼应【an adversative,oft.preceded by such conjunctions as虽然(thought)】
但是:(连词conj.)用在后半句话里表示转折,往往与“虽然、尽管”等呼应【used to introduce the next part of a sentence and indicate unexpectedness or exception】
倒是:(副词adv.)表示跟一般情理相反【used to indicate contrast to what is usu】
却:(副词adv.)表示转折,比“倒、可”的语气略轻【indicating a turn in meaning,weaker than 到or可】