Je vs. moi
In French, both je and moi translate to I or me, but they are not interchangeable. Their use depends entirely on their grammatical role within a sentence. Je is a subject pronoun, while moi is a stressed or disjunctive pronoun used in specific situations.
Je
A1This is the subject pronoun. It is always used directly before a conjugated verb to state who is performing the action. It becomes j before a verb starting with a vowel or a silent 'h.
Je parle français.
(I speak French.)
J'habite à Paris.
(I live in Paris.)
Je ne veux pas partir.
(I do not want to leave.)
Je suis fatigué aujourd'hui.
(I am tired today.)
Je pense que tu as raison.
(I think that you are right.)
Moi
A1This is the stressed pronoun. It is used for emphasis, when the pronoun stands alone without a verb, in comparisons, after prepositions, or as part of a compound subject.
Moi, je préfère le café.
(Me, I prefer coffee.)
Qui a mangé le dernier gâteau ? Moi !
(Who ate the last cake? Me!)
Il travaille avec moi.
(He works with me.)
Ce livre est pour moi.
(This book is for me.)
Mon frère est plus jeune que moi.
(My brother is younger than me.)
Toi et moi devrions regarder un film.
(You and I should watch a movie.)
C'est moi qui ai appelé.
(It is me who called.)
Elle aime la glace. Moi aussi.
(She likes ice cream. Me too.)
Summary
In short, the fundamental difference is grammatical. Use je as the subject right before a verb (e.g., Je vais - I go). Use moi in almost all other cases: for emphasis (Moi, je...), after a preposition like avec or pour (avec moi), in a comparison (plus grand que moi), or as a standalone answer (Qui ? Moi !).