Both は and が can sometimes seem possible, but they do different things.
In this sentence, は is natural because the speaker is talking about the ending of this movie as the topic and then describing it.
If you used が, it would sound more like identifying or emphasizing that the ending is the thing that is sad:
This is not impossible, but は is more natural for a simple descriptive statement like this.
悲しい is an i-adjective, meaning sad.
In Japanese, i-adjectives can already function as predicates:
Adding です makes it more polite:
So です here adds politeness. It does not turn 悲しい into a noun or change its basic meaning.
Because 悲しい is an i-adjective.
With i-adjectives:
You do not add だ after an i-adjective.
Compare:
So:
最後 literally means last, final, or end, depending on context.
In this sentence, 映画の最後 naturally means:
So even though 最後 by itself is not exactly the same as the English word ending in every context, here that is the most natural meaning.
Yes, compared with English, Japanese word order is different.
English:
Japanese:
Japanese often puts:
So the important descriptive part, 悲しいです, comes at the end.
The spaces are only for learning purposes.
Normal Japanese is usually written without spaces:
この映画の最後は悲しいです。
Beginner materials often add spaces to make it easier to see the parts:
But in real Japanese writing, you normally would not write it with spaces.
Not exactly. The sentence specifically says that the ending is sad, not necessarily the whole movie.
If you wanted to say This movie is sad, you would say:
That changes the topic from the ending to the movie itself.
Yes. In Japanese, 悲しい can describe not only a person’s feeling but also something that causes sadness or has a sad emotional quality.
So 悲しい ending is perfectly natural in Japanese:
In English, we often distinguish:
Japanese 悲しい can cover both kinds of usage.
Yes, both are possible, but they have slightly different nuance.
この映画の最後は悲しいです。
A natural neutral statement: The ending of this movie is sad.
この映画の最後が悲しいです。
Puts more focus on the ending as the thing that is sad.
この映画は最後が悲しいです。
As for this movie, the ending is sad.
This structure is also natural, especially if you are talking about the movie as a whole and then specifying what about it is sad.
So the original sentence is a very standard and natural way to say it.
You can simply drop です:
この映画の最後は悲しい。
That is the casual/plain form.
Compare:
Both mean the same thing, but the first is more polite.