"I slept."

Translation: Ich habe geschlafen.

[Pepe973392]

Just told me it was "Ich geschlafen" two questions ago

[az_p]

Either you misunderstood or there was a bug in the other example. The only correct answers here are:
ich habe geschlafen
ich schlief

Both mean the same thing in standard German, but the first is more likely in speaking and the second in writing. The first one needs the 'helping verb' to make sense.

[Rolfi_Popolfi]

It's also regional, in the south they rather tend to say latter. I would rather have translated it literally to "ich schlief" to make the grammar clearer

[nic37750]

geschlafen is part of the Perfekt form which needs the verb "haben" (like I have slept - ich habe geschlafen, or I have written - ich habe geschrieben). "Schlief" is Präteritum (like I slept -ich schlief, or I wrote - ich schrieb).