Psalm 40:11 New English Translation

¹¹O Lord, you do not withhold^[a] your compassion from me.
May your loyal love and faithfulness continually protect me!^[b]

Psalm 40:11 tn Some (cf. NIV, NRSV) translate the verb as a request (“do not withhold”), but elsewhere in the psalms the second masculine singular prefixed form, when addressed to God and preceded by לֹא (loʾ), is always indicative in mood and never has the force of a prayer (see Pss 16:10; 22:2; 44:9 51:16-17; 60:10; 108:11; cf. NEB, NASB).
Psalm 40:11 tn In this line the psalmist makes the transition from confidence to petition (see v. 13). Since the prefixed verbal form in the preceding line is imperfect/indicative, one could take the verb in this line as imperfect as well and translate, “your loyal love and faithfulness continually protect me” (cf. NEB). However, the כִּי (ki) at the beginning of the next verse, if causal (“because”), is best understood as introducing a motivating argument in support of a petition. For this reason v. 11b is best taken as a prayer with the prefixed form being understood as jussive (cf. NIV, NRSV). For parallels to the proposed construction (jussive followed by כִּי + perfect introducing motivating argument), see Ps 25:21, as well as Pss 10:2-3; 22:8.

New English Translation (NET)