Adam Clarke on Psalms 86:11

Teach me thy way - Instruct me in the steps I should take; for without thy teaching I must go astray.

Unite my heart - יחד לבבי yached lebabi, join all the purposes, resolutions, and affections of my heart together, to fear and to glorify thy name. This is a most important prayer. A divided heart is a great curse; scattered affections are a miserable plague. When the heart is not at unity with itself, the work of religion cannot go on. Indecision of mind and division of affections mar any work. The heart must be one, that the work may be one. If this be wanting, all is wrong. This is a prayer which becomes the mouth of every Christian.