Spanish is my mother tongue, but after living many years in France I am more proficient in French. However most of my reading/writing/coding is done in English. Now I am learning Italian. You can intuitively understand most of it. However there are some funny traps.

False friends

Word Translation False friend
Anche También/Incluso Aunque
Attento Cuidadoso Atento
Bagnato Mojado Bañado
Brutto Feo Bruto
Campione Muestra Campeón
Chissà Quien sabe Quizás
Disegnare Dibujar Diseñar
Guardare Mirar Guardar
Guadagnare Ganar Guadañar
Incidente Accidente Incidente
Mai (non mente mai) Nunca (nunca miente) Más (no miente más)
Nuotare Nadar Notar
Posto Lugar Puesto
Pronto Listo Pronto
Provare Intentar Probar
Seder(e/si) Sentar(se) Ceder
Sentire Oír Sentir
Spalla Hombro Espalda
Tenere Mantener/guardar Tener
Topo Ratón Topo
Tuttavia Sin embargo Todavía
Voto Notas (evaluación) Voto (elección)

Completely different words

Italian Spanish French
Addiritura Incluso Même
Aggiornare Actualizar Mettre à jour
Anatra Pato Canard
Autista/Guidatore Conductor Conducteur
Bugia Mentira Mensonge
Buio Oscuro Sombre
Busta Sobre Enveloppe
Cattivo Malo Mauvais
Chiunque Cualquiera N’importe qui
Cibo Comida Nourriture
Compito Tarea Devoirs
Comunque En todo caso Dans tous les cas
Così Así/Tan Comme ça/Tellement
Dito Dedo Doigt
Ecco/Qui Aquí/Acá Voici/Voilà/Ici
Entrambi Ambos Les deux
Dovunque/Ovunque Pordoquier/Dondequiera Partout/Où
Fermare Parar Arrêter
Finché Hasta que Jusqu’à ce que
Fiume Río Fleuve
Gara Carrera Course
Ghiaccio Hielo Glace
Ginocchio Rodilla Genoux
Giù Abajo Bas
Gonna Falda Jupe
Guai Dificultad Difficultés
Imparare Aprender Apprendre
Maiale Cerdo Cochon
Mandare giù Tragar Avaler
Matita/Penna Lápiz/Esfero Crayon/Stylo
Mucca Vaca Vache
Niente/Nulla Nada Rien
Non è neppure/Neanche Ni siquiera/Siquiera Même pas
Noioso Aburrido Ennuyeux
Noleggiare Alquilar Louer
Pasto Merienda Repas
Portare Traer Ramener
Pranzo Almuerzo Déjeuner
Presto Pronto/Temprano Bientôt/Tôt
Occhiali Gafas Lunettes
Ogni/Ognuno Cada/Cada uno Chaque/Chacun
Ritenere Considerar Envisager
Roba (plural) Cosas Trucs/Choses
Sabbia Arena Sable
Scegliere Escoger Choisir
Schermo Pantalla Écran
Scherzo Chiste Blague
Scatola Caja Boîte
Scarpe Zápatos Chaussures
Schiena Espalda Dos
Scorsa Pasada Dernière
Sedia/Seggiola Asiento/Silla Siège/Chaise
Sottile Delgado Mince/Fin
Soffice Suave Doux
Sporco Sucio Sale
Spostare Mover Bouger/Déplacer
Tacchino Pavo Dinde
Tasche Bolsillos Poches
Tastiera Teclado Clavier
Tonda Redonda Ronde
Uccello Pájaro Oiseau
Vuota Vacía Vide

Different preposition usage

Multiple meanings

Verb auxiliaries

Why it has to be so irregular ?!

All latin languages I know have the same problem but being a native speaker you do not feel the pain that much. But if those were programming languages I think nobody would actually learn them because they are so full of WTF ?! To be usable the language must be redundant and words must be phonetically and graphically different from each other, but I think latin languages go too far in this direction.

But somehow this irregular forms give you more expresivity by changing a few letters like :

Conjugation