hello – nee-how
Written as nĭ hăo or ni hao
To remember that hello is pronounced nee-how (ni hao) in Mandarin Chinese, use the following mnemonic:
I said hello to the jogger with two broken knees. How (nee how) did she run this far?

Tones
1. Nee (knees) is a 3rd tone with ∪ as an accent.
However, nĭ hăo has two 3rd tones in a row so nee becomes a 2nd tone. See 3rd Tone, 3rd Tone Combination Rule.
Associate knees, a 2nd tone, with a balloon filled with helium or anything that goes up. See Tones and Mnemonics.

She was lifted up by balloons tied to her knees.
2. How (how) is a 3rd tone with ∪ as an accent.
Associate how, a 3rd tone, with a horse shoe or horse or smile. See Tones and Mnemonics.

How was her smile so big?
Remember
Nee = 2nd tone = ⁄
How = 3rd tone = ∪
Examples of nihao (hello) in sentences:
Ni hao, hen gaoxing jian dao ni. - Hello, nice to meet you.
Ni hao? Ni hai zai ma? - Hello? Are you still there?
Ni hao, ni cong nali lai? - Hello, where are you from?