I heard...
Expresses hearsay or reported information with 'I heard that...'.
This pattern is used to report something you heard from someone else. It is a casual way to say 'I heard that...' and is often used in conversation. The verb 聞く is in past tense (聞いた), focusing on the moment you heard the information. It is not used for things you know directly, only for secondhand information.
You can use it with any clause that ends in plain form. For example, with verbs: 行くと聞いた (I heard he will go); with い-adjectives: 安いと聞いた (I heard it's cheap); with nouns and な-adjectives, you need だ: 先生だと聞いた (I heard he's a teacher). The particle と marks what was heard.