I came across an awesome quote and have tried "mia komencanta mano" at translating it.

Justice must be tempered with mercy; otherwise, it is nothing more than revenge.

Justeco devu hardi kun kompatemo; alie, gxi estas neniu plu ol vengxo.

Thoughts?

English is not my language, so I just looked up "temper" in the Merriam-Webster dictionary.

It says:

1 tem·per transitive verb ˈtem-pər
: to make (something) less severe or extreme
: to cause (something, such as steel or glass) to become hard or strong by heating it and cooling it

But what does it mean in this quote? Justice must be hardened (malmoligi) or must be made less severe (malseverigi)?

Maybe like this:

La justico estu malseverigata per la kompatemo; alie ĝi estas nenio krom venĝo.

Oni malseverigu la justicon per la kompatemo; alie ĝi estas nenio krom venĝo.

Oni malseverigu la justicon per la kompatemo; alie ĝi estas nur venĝo.

I would personally translate this as "Oni malseverigu justecon per kompato; alie estas nur venĝo."

While some of my decisions (like changing the sentence from passive voice to active voice with the subject "oni," or omitting the pronoun "ĝi") are personal preference, you've make a few objective mistakes in your translation. Here are those that I have noticed:

• I've never seen "devu" before. I'm not very familiar with the volitional in everyday Esperanto use, but to me, putting the verb for "should" into the volitional seems redundant. It's like saying justice "must should" temper, imho.
  
• "Hardi" is a transitive verb. The subject of the sentence is the one doing the tempering, while the object is the one tempered. In your translation, justice is tempering something which you don't name, whereas the original quote says that justice is what needs to be tempered. In my translation, I added a subject. A more literal translation would read "justeco devas esti hardita/hardata." Edit: "Hardi" is actually the wrong verb. As pointed out by nornen above, it means to literally temper something - to make something harder. The English word can also mean "to make less severe," and that's the meaning in this quote. "Malseverigi" is a better fit.
  
• You use the wrong preposition for "with." "Kun" is the preposition for "along with." It makes it sound like justice and compassion are tempering something together. The English "with" in this sentence means "by means of," so you would use "per."
  
• I think "plu" is the wrong word for more in this situation. The difference between "pli" and "plu" is something I don't have down yet, but I'm pretty sure "plu" is an adverb, closer to English's "further." I did away with that whole phrase and replaced it with the word "nur," which means "merely." More literally, I think "ĝi estas nenio pli ol venĝo" is closer to what the original phrase means.

I hope this helps! A lot of these are the same kinds of mistakes I made when I was starting out, and translating my favorite books and quotes is part of how I improved!

Edit: Nornen's post made me realize that the literal meaning of "hardi" isn't the same as the use of the word "temper" here! Changed my response to reflect that.

Thanks for the help. I definitely like to try my hand at translating- otherwise I'll never improve.

Here's another one:

Whoever loves, let him flourish.
Let him perish who knows not love.
Let him perish twice whoever forbids love.

Kiu ajn amas, lin permesu flori.
Lin permesu perei, kiu ne scias amon.
Lin permesu perei dufoje, kiu malpermesu amon.

Altulo:Here's another one:

Whoever loves, let him flourish.
Let him perish who knows not love.
Let him perish twice whoever forbids love.

Kiu ajn amas, lin permesu flori.
Lin permesu perei, kiu ne scias amon.
Lin permesu perei dufoje, kiu malpermesu amon.

I would say:

Kiu amas, floru.
Pereu, kiu ne konas la amon.
Pereu duoble, kiu malpermesas la amon.

Thanks again!

Altulo:Here's another one:

Whoever loves, let him flourish.
Let him perish who knows not love.
Let him perish twice whoever forbids love.

Kiu ajn amas, lin permesu flori.
Lin permesu perei, kiu ne scias amon.
Lin permesu perei dufoje, kiu malpermesu amon.

The usual formula I’ve seen for this sort of thing is kiu x-as, tiu y-as.

nornen:English is not my language, so I just looked up "temper" in the Merriam-Webster dictionary.

It says:

1 tem·per transitive verb ˈtem-pər
: to make (something) less severe or extreme
: to cause (something, such as steel or glass) to become hard or strong by heating it and cooling it

But what does it mean in this quote? Justice must be hardened (malmoligi) or must be made less severe (malseverigi)?

Consider a set of weighing scales, on one side is revenge (punishment of the offender), on the other side is mercy (compassion toward the offender), justice is the balance point between these.

So in this context, justice is tempered (made less severe) by mercy.