How to avoid mistakes when communicating in an online forum?

Your original question, as an example, read: How do avoid mistakes when communicating online forum?

First, watch for spelling errors (to, instead of do in the original question).

Second, used complete sentences. In your question, you left out "in an" before the end of the sentence. Those little Words can change the meaning of a sentence. For example:

Bring the dog bed.

Bring the dog to bed.

Bring the dog to his bed.

Each has a different meaning. We need to little Words, like "to" to explain meaning.

Third, avoid "texting" abbreviations. They make people look uneducated, despite their real education.

Fourth, be brief if you can. But if you need to explain points, do so. Don't make people guess at your intent or thoughts.

Fifth, be courteous. You never know when brain and fingers might hit the wrong keys; a nasty E-mail could go to your spouse, your professor, your boss....or even to your grandma. If you're courteous all the time, you won't ever have to worry about someone seeing you at your worst.