first – dee-ee
Written as dìyī or diyi
To remember that first is pronounced dee-ee (diyi) in Mandarin Chinese, use the following mnemonic:
She was the first to defeat the giant eagle (dee-ee).

Tones
1. Dee (defeat) is a 4th tone with \ as an accent.
Associate defeat, a 4th tone, with lowering things like trip or slide down. See Tones and Mnemonics.

His enemy fell off the cliff when he was defeated.
2. Ee (eagle) is a 1st tone with — as an accent.
Associate eagle, a 1st tone, with a string or tie or tying or knot. See Tones and Mnemonics.

The eagle was wrapped up in string.
Remember
Dee = 2nd tone = ⁄
Ee = 1st tone = —