Learn Hardcore Japanese: eiga ha omosiroi desu.

は is the topic‐marking particle. It tells the listener what you’re talking about—映画 (the movie) is the topic—and then you make a comment about it (that it’s interesting). It doesn’t mark the grammatical subject (that would be が), but rather introduces the theme of your statement.

Why does 映画 appear at the beginning of the sentence?

Japanese tends to follow a topic–comment structure. You first introduce the topic with 映画は, then you state your comment or observation: 面白いです. So putting 映画 first tells your listener “as for the movie…,” then you complete the thought.

What kind of word is 面白い, and how does it behave grammatically?

面白い is an i-adjective (ending in い). I-adjectives can function as predicates on their own (they can mean “is X”) and can also directly modify nouns (e.g., 面白い映画 “an interesting movie”). They conjugate like verbs, so you’ll see forms like 面白くない (negative) or 面白かった (past).

Why is です added after the i-adjective 面白い?

Adding です makes the sentence polite. I-adjectives already carry the meaning “is X,” so 面白い alone means “(it) is interesting.” Attaching です doesn’t change the basic meaning; it just raises the politeness level. In casual speech, you’d often just say 面白い.

Why can’t we say 面白いだ instead?

I-adjectives already include their own predicative function, so they never pair with だ (the plain copula). だ goes with nouns and na-adjectives but not with i-adjectives. If you want plain speech you simply say 面白い; to be polite you say 面白いです.

How would you say the same thing in casual (plain) Japanese?

You would drop です and say:
映画は面白い。
If context is clear, you might even omit 映画は and just say 面白い to mean “It’s interesting.”

Could we use が instead of は—as in 映画が面白いです?

Yes, you can. 映画が面白いです emphasizes that the movie (as opposed to something else) is interesting or presents it as new information. Using は makes it a known topic and adds a neutral or contrasting feel (“as for the movie…”). The nuance shifts depending on which particle you choose.

Are the spaces shown between words required in real Japanese writing?

No. Japanese text normally doesn’t use spaces; they were added here for clarity to show each element (映画　は　面白い　です). In actual writing you’d see 映画は面白いです without spaces.