Usos de "visto": "yo he visto", "él ha visto", "nosotros hemos visto", "ellos han visto" Usos de "vi": "yo vi", "el vio", "ella vio", "nosotros vimos", "ellos vieron" "He visto" es voz pasiva, "vi" es voz activa

They are both past tense but I think the second example is supposed to be "Yo he visto un gato" i.e. perfect tense "I have seen a cat", implying something more recent than "Yo vi un gato."

Like the previous commenter said, "visto" is something we call "el participio" of the verb "ver". It's analogous to the participle in English: if you have the infinitive "to see", the participle is "seen". So, in short, they are both past tense but they don't refer to the same time in the past. "Yo vi un gato"= "I saw a cat" whereas "Yo he visto un gato"= "I've seen a cat".

"Yo vi un gato." = "I saw a cat" "Yo visto un gato'' It is wrong ''Yo HE visto un gato'' = here is correct in english means '' I have seen a cat'' Also the other compound tenses: Pluscuamperfecto (Yo había visto un gato) Futuro perfecto (Yo habré visto un gato) Condicional perfecto (Yo habría visto un gato) Etc.. see the rest here http://www.wordreference.com/conj/EsVerbs.aspx?v=ver

He visto= I have seen Vi= I saw Yes both are past. But one is simple past and the other present perfect. Then the relevant quiestion would be WHEN did you see the cat? If today Hoy he visto un gato. If yesterday: Ayer vi un gato The Spanish grammar works on this aspect in a very similar manner as the English BUT people tend to find their own way. What will it be right in English: I saw a cat or I have seen a cat? It is the same case. But more precise, would you say: I just saw a cat or I´ve just seen a cat? Any way, here is the grammar for Spanish: The pretérito perfecto (present perfect, he visto) and the indefinido (simple past,vi) are used for completed actions in Spanish. The indefinido is used for actions that take place further back in the past, while the pretérito perfecto is used for actions that take place in a time period lasting until the present (este …) or for actions that have an influence on the present. https://espanol.lingolia.com/en/grammar/tenses-comparison-indicative/perfecto-indefinido