The "I" is here built into the verb, the mentiar, not the est. Mentiar is specifically the first-person singular form of "to lie" in the tense, mood, and voice that is wanted here.
When, in Latin, the subject of a sentence is a personal pronoun ("I", "thou", "he", "she", "it", "we", "you", "ye", "they") it is usually not expressed as a separate word; the idea of the subject is conveyed entirely by the verb-form. It is possible to state such a pronoun separately, but to do so places considerable emphasis on the subject-- puts it in italics, so to speak.
Mentior ("I lie"), as I mentioned, is a special kind of verb called a deponent, which can be confusing to beginners-- OK, sometimes to non-beginners as well. So for purposes of illustration, let us (as is traditional) use the simpler amo ("I love"):
amo = I love
amas = thou lovest (you (singular) love)
amat = he /she /it loveth (love)
amamus = we love
amatis = you (plural) love
amant = they love
The same kind of thing applies in other tenses, voices and moods. For example:
amavi = I have loved
amavisti = thou hast loved, you have loved
amavit = he /she /it hath loved (have loved)
amavimus = we have loved
amavistis = you have loved
amaverunt = they have loved.
The Latin pronoun equivalent to "I" is ego. But to say ego amo rather than just plain amo would be like saying: "It is I who love", or "I (and not some other) love".
In the same way the "I" is "built into" our mentiar. OK?
