As Amanda said both are correct, it depends on the context. It depends on the tense. "Forget" can also apply in the future as well. But "forgot" is always in the past. "I forgot to tell you I was learning Chinese" is a correct sentence. "I fear I will forget my keys tomorrow" is a correct sentence.

"I forget" is present simple. "I forgot" is past simple. Both are correct. I forgot my keys yesterday. <--- past action I usually forget my keys. <--- routine or habit