Exegesis:

embainontos autou eis to ploion ‘as he was embarking’: the present participle is to be thus translated.

embainō (cf. 4.1) ‘enter,’ ‘go into.’

parekalei auton ho daimonistheis ‘he who had been demon-possessed begged him’: the aorist participle describes the past condition of the man.

hina met’ autou ē ‘that he might stay with him’: i.e. accompany Jesus about as a disciple (cf. 3.14).

hina ‘that’: describes the content of the request, not the purpose of it.

Translation:

Getting into the boat may be described in various ways: ‘climbing into…,’ ‘going down into…,’ ‘jumping into….’ One should employ the typical manner of describing the boarding of this type of small craft.

Begged him that he might be with him may result in obscurity unless some of the pronouns are made more specific, e.g. ‘Begged Jesus that he could remain with him.’ For languages in which this type of expression must be in direct discourse, the following pattern may be followed: ‘begged Jesus, I want to stay with you.’

In a number of translations it has been found that the last clause of verse 18 was understood as a request by the former demoniac that Jesus would remain there in the land of the Gerasenes, a not unusual type of interpretation, but one which should be carefully avoided.

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 .