• Basic forms
• 負ける — to lose; be defeated / base form
• 負けます — (I) lose / masu form
• 負けた — lost / ta form
• 負けて — lose and …; please lose (contextual) / te-form
• 負けない — don’t lose / negative form
• 負ければ — if (someone) loses / conditional
• 負けろ・負けよ — lose! (rare command) / imperative
• Other common forms
• 負けたい — want to lose (rare) / 〜たい form
• 負けられる — can lose; can afford to lose / potential
• 負けられた — could lose (rare usage) / past potential
• 負けられない — cannot lose; must not be defeated / negative potential
• More
• 負けよう — let’s give in; let’s lose (contextual nuance) / 〜よう form
• 負けなかった — didn’t lose / past negative
• 負けている — is losing; is being defeated / progressive

負ける（まける） is an ichidan verb. Drop る to add endings such as ます, た, て, ない. Used for games, sports, competitions, and figuratively as “to give in” or “to yield,” as in 誘惑ゆうわくに負ける (give in to temptation).