What is the difference between I learned that song and I’ve learnt that song? I’m really confused between past tense and past participle.

Hint: Past tense and past participle are two different forms of verb and have different meanings and uses. In this case, I learned that song is past tense and I’ve learnt that song is past participle.

Complete answer:
The simple past is a verb tense used to describe events that occurred or existed in the past.
For example- I learned that song.
The sentence implies that the song was learned in the past. (It does not talk about whether the learning process was completed or not.)

The past participle is a verb tense that is used to describe events that occurred or existed in the past and were completed or finished.
For example- I have learnt that song.
The sentence implies that the song was learned in the past and the process was completed.

The past tense is a tense, while the past participle is a verb form that is used in the past and present perfect tenses.
There is no tense in the past participle. It's a verb form that can't stand on its own. An auxiliary verb, such as "have" or "had," is required. As a consequence, the past participle is often used in compound verbs.

Note:
Past participle is often used as an adjective as well.
Past participle form of the verb cannot work as a finite verb without an auxiliary verb(had/was/were). When a past participle comes without an auxiliary verb, it generally works as an adjective in the sentence.
For example: -
i) He looked dull with his swollen eyes.
ii) I cannot work within the given time frame.