JoseK is correct that the meaning of emend is confined to textual alterations, and that amend can be broadened to include the general improvement of other things. Still, amend is mostly used in the sense of improving text. If you're talking about fixing a rip in a shirt, you would be better to just use the word mend; to amend the shirt would sound strange. The Constitution of the U.S. may be amended (as it has been) but if the actual document itself were damaged and required restoration I don't think anyone would say the team that did the work amended the Constitution.
When decline is used as a transitive verb, it means "to refuse" or "to say no to": We are declining your loan application. - I regret that I must decline your invitation. Declining a customer would be a bad business move; declining your customer base is simply ungrammatical. Probably bad business too.
When a sentient actor (a person, a corporation, an intelligent animal) is the subject of decline in an apparently intransitive sense, there is generally an implied object; I would call this a "virtually transitive" use: I offered him a job, but he declined (the job). - We offered the chimp a banana, but she declined (the banana).
When a non-sentient noun is used as the subject of decline, it means that that thing/resource/quality is becoming less, or less powerful: The puma population has been declining for the past few years. - Hari Seldon says that the Empire is declining.
When a thing is declining, or a person's health or power is declining, we can say that that thing or person is in decline. As soon as his team started losing, he went into a decline. - This country's been in decline ever since they raised the drinking age.
When decrease is used as a transitive verb, it means "to reduce the amount of": I'll have to decrease my donut intake, or else my chair will break.
Sentient actors don't decrease intransitively; you can't say He decreased.
When a non-sentient noun is used as the subject of decrease, it means that that thing/resource/quality is becoming less: The puma population has been decreasing for the past few years. but NOT Hari Seldon says that the Empire is decreasing.
A crucial difference between decline and decrease in this last case is that decline can be used to indicate a loss of power, influence, significance, etc., whereas decrease can only be used for a reduction in quantity. Thus you can say both The population is decreasing and The population is declining, but while you can say The Empire is declining, you cannot say The Empire is decreasing, since there's still only one Empire.
Best Answer
James Fernald, English Synonyms and Antonyms, twenty-first edition (1914) lists debase and degrade in a synonym group that also includes abase, bring low, cast down, depress, discredit, disgrace, dishonor, humble, humiliate, lower, reduce, and sink. Fernald offers this distinction between the two words:
Similarly, Webster's Dictionary of Synonyms (1942) identifies debase and degrade as members of a synonym group that also includes abase, demean, humble, and humiliate, and offers this commentary on the two words:
Much like Webster's, S.I. Hayakawa, Choose the Right Word: A Modern Guide to Synonyms (1968) links debase and degrade to the words abase, demean, disgrace, humble, and humiliate:
I think that the last sentence in the Hayakawa excerpt is far more on point than any effort to impute a basic division in meaning that applies debase to things and degrade to people. Debasement is certainly a lowering, but it may be asserted at the end of a dispassionate analysis. Degradation, on the other hand, more often has a moral dimension, including a reaction of moral repugnance.
Still, I advise against treating these two words as if they were distinct in compass when properly used. In reality, the two words have a tremendous amount of overlap and even interchangeability—as do they and allied words such as demean, discredit, and disgrace—in everyday speech. The reference works that I've cited here do a nice job of identifying trends and tendencies in historical usage (and to some extent in contemporary usage), but they point to nuances, not to fundamental differences in meaning or usage.