Lieutenant is a word of French origin, and it is read as /lefˈtenənt/.
I’ve never learnt French and I don’t know anything about French reading rules, unfortunately. But I guess the English reading is related to the original French one.
Why is “lieutenant” read as /lefˈtenənt/ when there is no f in the word?
Nobody is sure.
/lefˈtenənt/ is indeed the British English pronunciation of lieutenant, but not in American English.
The OED says:
The origin of the β type of forms (which survives in the usual British
pronunciation, though the spelling represents the α type) is difficult
to explain. The hypothesis of a mere misinterpretation of the graphic
form (u read as v ), at first sight plausible, does not accord with
the facts. In view of the rare Old French form luef for lieu (with
which compare especially the 15th cent. Scots forms luf- , lufftenand
above) it seems likely that the labial glide at the end of Old French
lieu as the first element of a compound was sometimes apprehended by
English-speakers as a v or f.