Learn Dutch: Ik zou dat erg waarderen. - I would really appreciate that.

This is a standard Dutch rule for sentence structure. When you use a helper verb like zou (would), the main action verb (waarderen) is pushed to the very end of the sentence in its full dictionary form (the infinitive).

I thought erg meant "bad". Why does it mean "really" here?

You are right that erg can mean "bad" or "terrible" when used as a regular adjective. But when you use it to describe a feeling, adjective, or action, it means "very" or "really". So erg waarderen simply means "to really appreciate" or "to appreciate very much".

Could I use het instead of dat here?

Yes, you can! Ik zou het erg waarderen translates to "I would really appreciate it." Using dat (that) just puts a little more emphasis on the specific thing someone is doing for you compared to the neutral het.

How would this sentence change if I wanted to say "We would really appreciate that"?

Because we (we) is plural, the helper verb zou changes to its plural form: zouden. The rest of the sentence stays exactly the same, making it: We zouden dat erg waarderen.