Psalm 18:17 He delivered me from my strong enemy, and from them which hated me: for they were too strong for me.
Can for in this verse be translated because?
Did God deliver David because his enemies were too strong for him?
The word "for" (כִּי) can mean "because"; however, in this case, there is no causal meaning. Here it functions simply as a relative pronoun (the first definition from Gesenius' lexicon from Der Übermensch's answer).
I think the NIV translation renders it well:
He rescued me from my powerful enemy, from my foes, who were too strong for me.
The reason why God saved David is given with a causal meaning later on in the psalm:
He brought me out into a broad place;
he delivered me, because (כִּי) he delighted in me.
The Lord rewarded me according to my righteousness;
according to the cleanness of my hands he recompensed me.
For (כִּי) I have kept the ways of the Lord,
and have not wickedly departed from my God.
For (כִּי) all his ordinances were before me,
and his statutes I did not put away from me.
I was blameless before him,
and I kept myself from guilt.
Therefore the Lord has recompensed me according to my righteousness,
according to the cleanness of my hands in his sight. (vv. 19-24, NRSV)
Yes, both the Hebrew conjunction כִּי and its Greek equivalent ὅτι1 can be understood as “because.”
Gesenius on the Hebrew word כִּי (ki),2
On the English word “for,” Oxford English Dictionary states,3
Introducing the cause of a fact, the statement of which precedes or follows: Because.
17 He delivered me from my strong enemy and from those who hated me, because they were too strong for me.
What follows “for” (or “because”) is the reason that God delivered the psalmist.
Franz Delitzsch wrote on Psalms 18:17,4
כּי introduces the reason for the interposition of the divine omnipotence, viz., the superior strength of the foe and the weakness of the oppressed one.
Delitzsch, Franz. Commentary on the Old Testament. 1900. Reprint. Vol. 5. Trans. Bolton, Francis. Grand Rapids: Eerdmans, 1988.
Gesenius, Heinrich Friedrich Wilhelm. Gesenius’s Hebrew and Chaldee Lexicon to the Old Testament Scriptures. Trans. Tregelles, Samuel Prideaux. London: Bagster, 1860.