Exegesis:

ou gar sunēkan epi tois artois ‘for they did not understand about the loaves’: i.e. from the multiplication of the loaves, in the feeding of the multitude, they should have gained insight (into the person of Jesus).

all’ ēn autōn hē kardia pepōrōmenē ‘but their heart was hardened.’

kardia (cf. 2.6) ‘heart,’ ‘mind,’ ‘thinking.’

pōroō (8.17; cf. pōrōsis 3.5) ‘harden,’ ‘petrify’; when used of ‘heart’ it means ‘to grow (or, make) dull,’ ‘blind,’ ‘obtuse.’

Translation:

Some translations of the expression they did not understand about the loaves have meant little more than ‘they did not remember about the loaves’ or ‘they did not think about the loaves.’ However, the meaning here is much more. What the Gospel writer implies is that they did not understand the implications of the miracle. If available terms rendering understand are inadequate in the receptor language, one may need to expand the clause somewhat, e.g. ‘they did not know what it meant when Jesus divided the loaves’ or ‘they did not recognize the meaning of the feeding the people with the loaves.’

There are relatively few languages in which one can say ‘their hearts were hardened’ or ‘hard’ and at the same time preserve the meaning of the original at this point. If translated literally this expression would have the following meanings in various languages: ‘endurance’ (Sayula Popoluca), ‘brave’ (Tzeltal), ‘doubt’ (Piro), ‘bad character’ (Chicahuaxtla Triqui) and ‘courage’ (Shilluk). On the other hand, one can always speak of the characteristic denoted by the phrase their hearts were hardened, e.g. ‘they have hard heads’ (Chicahuaxtla Triqui), ‘their ears do not have holes’ (Shipibo-Conibo), ‘they do not have pain in their hearts’ (Tzeltal).

Hardened indicates primarily a state of being resulting from a process, not a specific process requiring the identification of the particular agent. The Greek has reference to the condition of the hearts, not the process by which they become hardened.

Quoted with permission from Bratcher, Robert G. and Nida, Eugene A. A Handbook on the Gospel of Mark. (UBS Handbook Series). New York: UBS, 1961. For this and other handbooks for translators see here .