You can say either “I just saw” (past simple) or “I’ve just seen” (present perfect); there isn’t much difference in meaning between them, since in any case, “just” fixes the time of the event to the recent past. There’s a slight difference in emphasis, but in most cases, expressions like: “I just saw the movie.”

I just seen (in your question) = This is incorrect. Some uneducated native speakers may say this, but it is completely ungrammatical. You should never use this form.

‘Saw’ is the past tense of the word ‘see’ while ‘seen’ is the past participle. Typically, ‘saw’ comes immediately after the noun or pronoun. For instance, “Steve saw the movie.” ‘Seen’ is never used as a standalone verb and is generally accompanied by words such as ‘have’, ‘had’, ‘was’, among others.

[M] [T] She told him that she had seen me there last week. [M] [T] Seen from the sky, the river looked like a huge snake. [M] [T] Seen from a distance, the big rock looks like an old castle. [M] [T] She had never seen New York before, so I offered to show her around.

Saw is the PAST TENSE of the verb see, and usually comes immediately after NOUNS and PRONOUNS. Seen is the PAST PARTICIPLE of the VERB see. Generally, seen is used alongside have, has, had, was or were in a sentence to make COMPOUND VERBS.

Stars can be seen at night. In the context of his religion, his actions can be seen as reasonable. Inadequate leisure time can be seen as a limiting factor that prevents innovation and experimentation. The high building can be seen from the window.

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