Exegesis:

humeis ‘you yourselves’: the pronoun is emphatic (cf. Rawlinson).

blepete (cf. v. 5) ‘beware!’ ‘watch out!’

proeirēka (only here in Mark) ‘I have foretold,’ ‘I told (it) beforehand.’

Translation:

Take heed does not refer to the people’s paying close attention to what Jesus was saying at that time, but to their being aware of what was going to happen in the future, i.e. ‘be on your guard against,’ ‘be careful,’ or ‘watch out for what will happen.’

Beforehand may require expansion into the form of a clause, e.g. ‘before these things happen’ or ‘before these events take place.’

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 .