If you hear "donkah", it's a schwa:
'ils' and 'il' DO sound the same unless there's liaison: Ils sont (they are) = "eel so(nasal)" V.S. Ils ont (they have) = "eel_zo(nasal)". The letter 's' is carried over like a 'z' sound in 'ils ont' because it links to the following vowel (liaison). In the case of 'boire', il boit, ils boivent. There is NO liaison because of the consonant "b". The way you know that it's 'ils' is because the verb is in the plural 'boivent'; so, the subject must be plural as well (ils). You can't write 'il boivent'. I hope this helps.
This might be an extremely stupid question, but if you listen to it slowly, still how can you tell the difference between "il mange" and "ils mangent"? And one other stupid question, would it make grammatical sense to say "il mange donc ils boivent" like "he eats so they drink"?
The sentence itself lacks in any logic anyway. However, with homophones, you can understand "il/ils" twice and theoretically combine them as yo like.
But again, teaching "donc" with so few words available to build a meaningful sentence ends up producing artificial meaning and grammar options.
It is stylistic. I personally use alors in short sentences that express sorrow/regret or... oh goodness its hard to explain... it can start a sentence in a way that is not at all like 'so' in English... I also tend to use it for more abstract cause and effect... donc I tend to use with more closely related cause and effect and sometimes as a filler, as 'so' is used in English.
I am not fluent in French though, so I don't know if my use would reflect that of a native speaker, but I am fairly certain that there is no strict grammatical difference between the too.
I think I might use 'alors' like 'thus' and 'donc' like 'so'.
Il mange et ils mangent sound exactly the same, therefore you have to make it up in context of, here in this sentence is it normal you pass,there is no information where you can be sure it is singular or plural, only in the second piece of the sentence "ils boivent" you can hear the gender I hope this helps
One of the options of the meanings for "donc" is "consequently". Therefore, my sentence for the translation of: "Ils mangent, donc ils boivent" must be accepted. Many times Duolingo shows options for your translation, but when you use them, they are not accepted. This is not fair.