Ttgjyuk

When do we add an hyphen (-) to a complex adjective word?

Here's are a few examples:

This is an Xbox-compatible game.

This is a Creation-Kit-compatible 3d asset.

This is a SkyRe-compatible Skyrim mod.

More often than not all these examples are used without a hyphen or hyphens. Is this a case of people making a grammatical mistake or are both forms completely correct and it's just a matter of preference?

This is not a question of grammar, but of style. Writers use hyphens with compound adjectives to avoid ambiguity so that the reader does not have to read and re-read a sentence to garner the meaning from it.

Consider:

John was a white bearded man.

Someone might try to parse this sentence at first to mean he was a white man who had a beard.

John was a white-bearded man.

This makes it quite clear that John was a man with a white beard, not a white man with a beard.

Robusto's answer is correct, I'm just adding another thought.

You asked:

Is this a case of people making a grammatical mistake or are both forms completely correct and it's just a matter of preference?

Robusto didn't quite address that question head on. While there is a lot of flexibility in punctuation, I would say any professional editor worth his or her salt would correct 'white bearded' to 'white-bearded'. Leaving out the hyphen is not optional.

Yes, in informal contexts, writers very often leave out the hyphens, either because they are unsure how to use them, or they forget. But that's not the same as saying they are optional. They are making a mistake which can lead to misunderstandings, and knowing how and why to use hyphens in compound adjectives places you at an advantage.

Finally, often with punctuation, we say "well, it doesn't exist in spoken English, so is it really required?"

But in spoken English there is an audible difference between

The white, bearded man.

and

The white-bearded man.

It's subtle, but it's there, and it makes all the difference to the interpretation of the sentence.