slay / slew / slain / slayed
The past tense and past participle of slay.
The past tense of slay is slew, and the past participle is slain. Slayed is the past of the slang slay, meaning to impress or to perform superbly.
Examples
Use The raiders slew the sentries before dawn.
Use Two guards were slain at the gate.
Avoid The raiders slayed the sentries before dawn. (Change slayed to slew.)
Avoid Two guards were slayed at the gate. (Change slayed to slain.)