Learn Hardcore Japanese: wasuremono ha kuroi kutu desita.

忘れ物 is a compound of the verb stem 忘れ- (from 忘れる, “to forget”) and 物 (“thing”). So it literally means “a thing that was forgotten,” i.e. a “forgotten item” or “lost item.”

What role does は play in 忘れ物は黒い靴でした and why not が?

The particle は marks 忘れ物 as the topic of the sentence—the thing we’re talking about. Using は says “as for the forgotten item, (it) was black shoes.” If you used が it would mark 忘れ物 as the grammatical subject and put focus on identifying it as new information (e.g. answering a “what was lost?” question). は simply sets the stage to describe it.

Why is there no particle between 黒い and 靴?

Because 黒い is an い-adjective and directly modifies the noun 靴. In Japanese, い-adjectives attach to nouns without any extra particle.

What’s the difference between 黒い靴 and 黒の靴?

Both mean “black shoes,” but:

黒い靴 uses the い-adjective 黒い to describe the color directly (“shoes that are black”).
黒の靴 uses the noun 黒 with the genitive の, almost like saying “shoes of black.” It’s grammatical but slightly more formal or literary, and you’ll hear 黒い靴 much more often in everyday speech.

What part of speech is 黒い?

黒い is an い-adjective (形容詞). It behaves like a verb in that it can conjugate for tense (e.g. 黒くない, 黒かった, 黒くなかった).

Why is でした used instead of だった, and what nuance does it add?

でした is the polite past form of the copula です, while だった is the plain past. Using でした makes the sentence more polite (suitable for strangers, customers, etc.). だった would be casual and likely used among friends or in informal writing.

Why doesn’t the sentence end with a verb like 忘れる?

This isn’t describing the act of forgetting—it’s an equational sentence using the copula です/でした to say “X was Y.” There’s no verb of action here; it simply links the topic (忘れ物) to its description (黒い靴でした).

How would you ask “What was the forgotten item?” in Japanese?

You would say 忘れ物は何でしたか？

何 means “what.”
でしたか turns your statement into a polite past-tense question: “As for the forgotten item, what was it?”

How would this sentence change if you describe the item in the present tense?

You’d use the non-past copula です instead of でした.
“忘れ物は黒い靴です。”
This says “The lost item is black shoes,” as if you’re identifying it right now.

Can you say 忘れ物が黒い靴でした? If yes, what nuance difference is there from 忘れ物は黒い靴でした?

Yes, it’s grammatically correct. が marks 忘れ物 as the subject presenting new information—more like answering “What was the lost item? It was black shoes.” Using は instead presents 忘れ物 as the known topic and then describes it. In practice, both can be used to answer “What did you lose?” but は feels a bit more neutral or declarative, while が feels more like you’re reporting newly discovered facts.