Given to translate ''Ils arrivent!'', my gut reaction was ''They're here!''. The alternative ''They arrive!'' seems very unnatural to me.
You may be monitoring someones progress where you say they arrive to mean something understood like they arrive at the airport which is a couple of hours of customs and a two hour drive before you are in a position to they are here.
Of course, it would sound more natural to say they arrived but we are not at the point of past tense in Duo lessons.
I don't think you can translate the French as "They are arriving" can you? In this case, it would mean they are in the process of arriving, perhaps their plane is approaching? It seems in French the meaning is limited to the actual moment of arrival?
French people I know use arriver to mean 'coming' or 'on the way', rather than actually arriving. If I'm meeting someone and phone to see where they are, j'arrive means they've probably left their house but might not be anywhere near where I'm waiting for them yet.
French has no continuous tenses, so present tense is translated to continuous tense in English, or not, based on what works best. In English, pretty much the only way to translate this, as a fragment at least without a dependant clause, is in the continuous - 'They are arriving'. Even then it would probably not be overly common - they are either here or they aren't. It would more likely be used to indicate the near future (futur proche): They are arriving at 7 pm tomorrow.
"Arriver" can also mean "happen," right? Like, j'arrive à le parler (I happen to speak it).
"J'arrive à le parler" is not exactly "I happen to speak it" but "I succeed in speaking it".
But still, yes, "arriver" can mean "to happen" like if you miss your bus and says "oh no, it always happens to me" = "oh non, ça m'arrive tout le temps".
True, but only in singular with the impersonal pronoun "il" or "ceci/cela/ça".
You should here a "z" sound (for the liaison between the final "s" of the pronoun and the vowel of the verb) : "IL_Z_ARIV", don't you?
Please tell me how I can tell the difference between "Il" and "Ils" in the spoken version. With just the two words I could not tell the difference between singular and plural with either and there were no other verbal cues to help.
The difference is heard in the plural version through the liaison: il-Z-arrivent
Following Sitesurf's answer, you wouldn't hear any difference when there is no liaison, like "il mange" / "ils mangent".