What is the difference between the affixes fi- and -aĉ?

Question

I've understood that both the prefix fi- and the suffix -aĉ means something bad. What are the differences of these two? Are there differences in how they are defined? In how they are used?

Answer 1

Put simply, "-aĉ-" is the one that means "Bad, of poor quality", whereas "fi-" actually means "Immorral/corrupt".

For example,

• A "Fiprezidento" would be a corrupt president, say taking bribes from Human Rights abusers.
• And a "prezidentaĉo" would be an otherwise nice person that just doesn't know how to do the job.

Answer 2

PMEG explains that fi is not a "real" affix, while aĉ is. Then it goes on and says, talking about the difference between them:

Prefikseca fi kaj la sufikso AĈ estas similaj. Fi estas pli subjektiva. Ĝi esprimas indignon kaj malŝaton. AĈ estas pli objektiva. Ĝi montras maltaŭgecon kaj malbonan kvaliton. En la frua tempo, antaŭ ol AĈ aperis en la lingvo, prefikseca fi estis uzata en ambaŭ signifoj.

The prefix fi and the suffix AĈ are similar. Fi is more subjective. It expresses wrath and dislike. AĈ is more objective. It shows non-fittingness and bad quality. In earlier times, before AĈ appeared in the language, the prefix fi was used with both meanings.

These are a couple of examples from PMEG (from here and here) that I could match and that show the difference:

• domo → fidomo = domo, kie okazas abomenaj malŝatindaj aferoj (a house where abominable, hateful things happen)

• domo → domaĉo = malbona, kaduka, disfalanta domo (bad, dilapidated, collapsing house; e.g. a shack)

Answer 3

From Plena Ilustrita Vortaro:

Inter fi k aĉ estas tia diferenco, ke aĉ esprimas, ke iun aŭ ion oni juĝas malbonkvalita, netaŭga ks, dum fi esprimas negativan juĝon el morala vidpunkto.
(Between fi and aĉ, the difference is that aĉ expresses that someone or something is judged poor-quality, unsuitable, etc, while fi expresses negative judgement from a moral viewpoint.)

• Kial via frato daŭre fiparolas? indicates that your brother was saying bad things, perhaps that he "has a foul mouth", or maybe he's slandering someone

• Kial via frato daŭre parolaĉas? is an attack on his manner of speaking, perhaps he stammers a lot, or uses bad grammar, or maybe I'm just annoyed and I don't want to hear from him

• fi-kanto is an immoral/"dirty" song (hyphen added to distinguish from fik-anto, which means something rather different, although one might sing about a fik-anto in a fi-kanto)

• kantaĉo is a badly written song, perhaps the lyrics are corny, or the tune is dissonant

• fivarma akvo is water that is warm in an evil or unethical way (What is that?)

• varmaĉa akvo is water that is unpleasantly warm to be drunk, for example because taken from fountains (a far more useful meaning)