To analyze and spell the word <lieutenant> we suggest using “say to spell” and adding an extra syllable: lie u ten ant.
In this case IE is saying long /ē/ as in the word <field>.
Ask the students say " lie u ten ant” faster and faster. Notice how when we say /ē/ followed by /oo/ quickly, the sounds assimilate, and the long /ē/ is dropped.
The second is to consider ieu an advanced phonogram that says /oo/.
When teaching this word it is also helpful to consider the morphemes:
Lieutenant is the second in command. Lieu is French for place or position. Tenant means to hold. So a lieutenant holds the place for the superior in his/her absence.