jega eoje bon yeonghwaneun jeongmal jaemiisseosseoyo.

• 제가 = 저 (humble “I”) + subject marker -가, marking the doer of “saw” inside the relative clause.
• 저는 uses the topic marker -는, which would make 저 (“I”) the main topic of the whole sentence—unnatural here, since the real topic is 영화 (“movie”).
• Also, 제가 is the polite equivalent of the informal 내가; you wouldn’t use 내가 in this polite speech level.

• Literally: 어제 (yesterday) + 본 (past attributive form of “to see”) + 영화 (movie).
• This is a relative clause: the verb in attributive form (본) comes before the noun it modifies (영화), so 어제 본 영화 = “the movie that (I) saw yesterday.”

• To turn a verb into an adjective modifying a noun, Korean uses the attributive ending -ㄴ (for past) or -는 (for present).
• Stem of 보다 is 보.
  • Past attributive: 보
  • ㄴ = 본
    • Present attributive: 보
    • 는 = 보는
• So 본 영화 = “the movie (that I) saw.”

• -는 marks 영화 as the topic: “As for the movie (that I saw yesterday)…” It frames what you’re talking about.
• Using -가 would make 영화 the subject, slightly shifting focus to identification: “The movie (that I saw yesterday) was really fun.” Both are grammatical; -는 is more natural in personal anecdotes.

• 정말 means really or truly.
• Placing it before 재미있었어요 intensifies the statement: “It was really fun.”
• You could also use 진짜 for a more casual feel.

• The ending -었어요 is polite informal (common in everyday conversation with strangers or acquaintances).
• Casual (친구말): drop -요 → 재미있었어.
• Formal (높임말): use -습니다 → 재미있었습니다.

Yes. Korean often omits subjects or agents when context is clear. You can simply say:
어제 본 영화는 정말 재미있었어요.
The listener understands that “I” am the one who saw the movie.