Learn Italian: Il sacrificio è pesante, ma ne vale la pena. - The sacrifice is heavy, but it is worth it.

The particle ne means 'of it' or 'for it'. The full pronominal verb is valerne la pena (to be worth the trouble of it). Here, ne links directly back to il sacrificio. While you can say vale la pena as a general statement, adding ne makes your Italian sound much more natural by explicitly connecting back to the thing that is worth the effort.

What is the grammatical subject of vale in this sentence?

The subject is actually la pena (the trouble or effort). Literally, the phrase translates to 'the trouble is worth it'. Because la pena is grammatically the thing doing the action, the verb valere is always conjugated in the third-person singular when using this expression.

Does pena mean physical pain here?

No, here it translates closer to 'trouble', 'effort', or 'hardship'. Physical pain is usually expressed with the word dolore. The fixed expression valerne la pena carries the idea of an outcome being worth the struggle or sacrifice you put into it.

If I wanted to talk about this sacrifice in the past tense ('it was worth it'), how would the phrase change?