Learn Icelandic: Ég set steinseljuna á marineraða kjötið. - I put the parsley on the marinated meat.

Because it is acting as an adjective modifying a definite noun (kjötið), it must use the weak declension. The noun kjöt is neuter and is in the accusative case here. For a neuter singular word, the weak ending is always -a, changing the strong form marinerað to the weak form marineraða.

Doesn't the preposition á usually take the dative case? Why are marineraða kjötið in the accusative?

The preposition á governs the accusative case when it describes motion toward a destination, like placing the parsley onto the meat. If the parsley were already resting on the meat (indicating static location), á would take the dative. The noun would change to kjötinu, though the weak adjective actually remains marineraða, making the phrase á marineraða kjötinu.

How is the definite word steinseljuna formed from the base noun?

The base word is steinselja (parsley), which is a weak feminine noun. Because it is the direct object being put, it takes the accusative case and becomes steinselju. To say the parsley, we add the feminine accusative definite suffix -na, resulting in steinseljuna.

Is set present or past tense here? In English, 'put' can be either.