What is the past tense of suffix?

The word "suffix" doesn't have a past tense. It's a noun, not a verb. It describes an element added to the end of a word.

If you are thinking of a word related to the past tense, maybe you meant:

* Suffixed: This is the past participle of the verb "to suffix," meaning to add a suffix to a word.

* Suffixation: This is the noun form of "to suffix," referring to the act of adding a suffix.

Let me know if you have a different word in mind, and I'll do my best to help!