For I said, Hear me, lest otherwise they should rejoice over me: when my foot slippeth, they magnify themselves against me.
For I said, (Hear me) lest (otherwise) they should rejoice over me - or, 'For I speak (thus to thee; as stated in Psalms 38:15), lest,' etc.
When my foot slippeth, they magnify themselves against me - i:e., when any misfortune befalls me (Psalms 35:26).