tamen

tamen marks the clause it sits in as running against what has just been said. Latin usually postpones it rather than putting it first, so it typically lands second or third in its clause, after the word being contrasted. It is the standard answer to a concessive clause opened by quamquam, etsi or licet, where English would say "although ... nevertheless". As an adverb it takes no case and changes nothing about the words around it.