This is a difficult question to answer, because both aches and pains are subjective experiences - like colours - which you're unable to share, but assume everybody understands. I would have assumed that every language has words for both ache and pain, so a dictionary would tell you the difference in an instant. But, I would also assume you've done that, so your native language might not (I'm interested to find out what language that is).
An ache is a persistent discomfort, typically dull so that you can try to ignore it, but sometimes all-encompassing, yet not sharp enough to describe as pain. Your legs would ache after a tough run; you would not describe this as pain. You usually get a headache, not a head pain. You would suffer pain when you cut your finger, then experience an ache as the wound heals.
A pain is something more localised, often (but not always) short-lived, and something you'd be less able to ignore.
When you receive an injection, there is a pain as the needle goes in. During the following days, the surrounding area will ache.
There is considerable overlap between the two, and it would be quite acceptable to say "the ache in my shoulder is painful".
Poets and songwriters quite often speak of their heart aching. This fits well with a persistent sense of yearning or melancholy. If they said there was a pain in their heart, it would suggest a quite different emotion.
Describing pain and discomfort is difficult and subjective; I imagine in any language. When a doctor asks you how much something hurts, how can you explain in a reliable way?
Relevance is the more common form, according to grammarist.com:
Relevance vs. relevancy:
- There is no difference between relevance and relevancy. Though the latter is the older form, relevance is now preferred in all varieties of English. In this century, relevance is about ten times as common as relevancy in U.S. popular usage, and the gap is even wider in British, Australian, and Canadian sources.
Ngram relevance vs relevancy
Etymology:
- relevance (n.)
1733; see relevant + -ance. Related: Relevancy (1560s).
-ancy (suffix):
- word-forming element denoting quality or state, from Latin -antia, forming abstract nouns on past participle adjectives in -antem, appearing in English mostly in words borrowed directly from Latin (those passing through French usually have -ance or -ence; see -ance).
Best Answer
Suasive is an adjective that, in Linguistics (Grammar), "denotes a class of English verbs, for example, insist, whose meaning includes the notion of persuading and that take a subordinate clause whose verb may either be in the subjunctive or take a modal."
Persuasive is an adjective as well, that means being "good at persuading someone to do or believe something through reasoning or the use of temptation:
an informative and persuasive speech.
" OR "She was very persuasive!
"The difference, then, is that while the former denotes a grammatical class for verbs, the second is adopted the way you already know, with people, situations, etc.
EDIT NOTE: In the OED it says that a speech can be "suasive" but considering the OALD and my dictionary didn't have it, I supposed it was an old use or it fell into disuse. So I checked the Ngram on google and it confirmed what Billare said and what I was thinking.