I just read a sentence:
Three things in life that, once lost, hard to build up.
Can I rewrite it as:
Three things in life that are once lost hard to build up.
Is my sentence correct?
What are the functions of commas?
I think there is a word missing in the sentence, but not the one you think it is.
I would expect
Three things in life that, once lost, are hard to build up.
Once lost is a "small clause", equivalent to once they are lost; it is also common enough to qualify as an idiom.
Note also that this is not a full sentence, since everything after "that" is a subordinate clause. The full form would be
There are three things in life that, once lost, are hard to build up.
The writer is using an informal style, where they have left out the "There are". But I don't think anybody would leave out the later "are" in speech: I think this is just a mistake.
If you put in all the assumed words, you would have this:
(There are) three things in life that, once (they have been) lost, (are) hard to build (back) up.
You also asked "What are the functions of commas"
In this context the commas are a sign of a side remark, a parenthetical statement that is used to provide further information. If you were reading this sentence aloud you would have a slight pause for the commas and a possible inflexion change.
A simpler way to write the sentence would be to move this clause to the end of the sentence:
Three things that are hard to build up once lost.
English rarely uses different forms of words to convey the grammatical function of a word and add to its meaning which is common in many other languages. This can be confusing to speakers of other languages.
Writers often decide to write a sentence in a particular way so that the effect of the sentence is different, to introduce different tone or rhythm. This is common in other languages too but having different word forms can make the meaning much clearer.