Very negative:
* Awful: implies extreme unpleasantness.
* Dreadful: suggests something causing fear or disgust.
* Horrible: emphasizes the shocking or horrifying nature.
* Appalling: expresses strong disapproval and disgust.
* Abysmal: describes something extremely bad or low in quality.
* Disastrous: refers to something causing great harm or failure.
* Catastrophic: implies a devastating and widespread disaster.
Moderately negative:
* Bad: a general term for something unpleasant or undesirable.
* Poor: indicates low quality or inadequate performance.
* Nasty: emphasizes unpleasantness or offensiveness.
* Unpleasant: simply means something not enjoyable or agreeable.
* Miserable: suggests extreme sadness or discomfort.
Other synonyms:
* Lousy: informal and suggests poor quality or worth.
* Shocking: emphasizes something surprising and unacceptable.
* Outrageous: suggests something shocking and offensive.
* Disgraceful: implies something shameful or dishonorable.
The best synonym for "terrible" will depend on the specific situation you are trying to describe.