"She does not think but speaks."
Translation:Dia tidak berpikir tapi berbicara.
'tetapi'= 'tapi' = but.
'namun' = 'akan tetapi' = however.
There is a difference in usage.
In the formal style ('baku') the rules are
'tetapi' = 'tapi' ==> is used to connect two clauses in one sentence.
'tetapi'/'tapi' appears after a comma, and may not open the sentence.
'namun' = 'akan tetapi' ==> is used to connect two sentences.
'namun'/'akan tetapi' opens the sentence, and may not appear after a comma.