〜は〜が
A's B is… (describing a feature)
Topic は + part / aspect が + adjective
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A very common shape: the topic with は, then a part or aspect of it with が, then an adjective. 象は鼻が長い, "elephants have long trunks", is literally "as for elephants, the trunk is long".
は sets up who or what you're talking about; が picks out the feature. So 田中さんは背が高い is "Mr Tanaka is tall" (as for him, height is high).
English folds this into "has" or a single adjective, so learners reach for の: 田中さんの背が高い. That's grammatical, but it's a statement about the height, not about Tanaka.
The same pattern gives you 私は頭が痛い, "I have a headache", and この町は人が多い, "this town is crowded".
Examples
象は鼻が長いです。
Elephants have long trunks.
田中さんは背が高いです。
Mr Tanaka is tall.
今日は頭が痛いです。
I have a headache today.
この町は人が多いです。
This town is crowded.
姉は髪が長いです。
My older sister has long hair.