〜は ・・・です。
Your first sentence — “A is B.”

“As for …” — it points at what the sentence is about.
わたしは = as for me
Close the sentence with it. Negative? Swap in じゃありません.
ねこです = it’s a cat
Add か at the very end — instant question. No word-order change.
ねこですか = is it a cat?

わたしは ねこです。

“I am a cat.” — that easy.
3つの かたち
Same sentence, three endings: です (is), じゃありません (is not), ですか (is …?). Tap any example to hear it.
One thing the kana chart can't tell you: when は is doing this job — pointing at the topic — it is pronounced wa, not ha. わたしは = “watashi wa”. Anywhere else は is still ha (はい = “hai”). Tap the examples below and you'll hear it.
“A is B.” — state what something is.
“A is not B.” — the negative.
“Is A B?” — add か to ask a question.
クイズ
Read the English meaning, then pick the right ending.
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