"He will gladly compare them."
Translation:Er wird sie gerne vergleichen.
You don't have to. You can use them interchangeably. 'gerne' is slightly more formal but nothing to bother about.
"Er wird gerne sie vergleichen" is unfortunately for me an incorrect assortment of those words.
gerne should go close to the main verb. Pronouns should come immediately after the inflected verb.
The penny drops.
If you think of the sentence as He will + gladly compare them then it comes out as er wird + sie gerne vergleichen. .
Think of the verb as gladly compare and the auxiliary as he will, rather than he will gladly + compare.
My translation was: er wird gerne sie vergleichen. Why is this order not correct? Thank you!
any reason why "Er wird sie glücklich vergleichen" should not work here?
Yes, glücklich is state of being, Du bist glücklich, etc.
When used as an adverb, happily, gladly, etc. You have to use gern, or gerne.