To be more specific, you add an a to single-syllable adjective. And what they often forget to explain is that in the Celtic languages, like German but unlike French, it is only attributive adjectives, not predicative ones that change:
Tha na rudan mòra mòr
Les grandes choses sont grandes
The big things are big
This exercise was pretty pointless because we had never seen 'rudan' before, so we hadn't any way to know whether it was a singular word or plural. 'rudan' was not introduced us (it was in orange letters) in the exercise following this one. I think Duolingo got these two exercises the wrong way round.
To the contrary, this was a well-designed exercise as it was teaching you how to identify a plural.
First, of course, you are quite right that some words ending in -an are plural and some are singular. For example, clachan can mean either 'a wee stone' (with -an a diminutive suffix) or 'stones' (with -an the plural suffix). And to add to this confusion this plural word has led to another singular meaning of 'village, hamlet' since there would have been paving stones in the village and not elsewhere.
To take the present example, rudan means 'things' but the very similar radan means 'a rat'. So how do you tell if a word you don't know is plural? You can't always tell, but you can here - mòr means 'big' but mòra is the plural form. We add an a/e to single-syllable nouns when they are qualifying plural nouns (according to the spelling rule). So they may well have been giving you the opportunity, if they gave you thing and things as options, to work out that it was plural from the evidence provided. And hopefully rud 'thing', and its plural rudan have now been lodged in your mind.
I can't tell which unit you are doing but these plural adjectives should have been mentioned in the notes in this unit or a previous one.
When you hear a sound in a language you are not familiar with your brain tries to match it with a sound you know in your own language. So when you hear this sound you are comparing it with how you pronounce l and r in your own language/dialect/accent and deciding which is closest. Now l varies quite a lot, but r varies an enormous amount. For me (Southern English) I pronounce r without tapping my tongue on my teeth or palate (so I am called 'non-rhotic'). I hear an r here. But if you are strongly rhotic, where the tongue touches once (a tap), twice (a double tap) or more than twice (a trill) then the sound here will not resemble an r so it could be, to you, more like an l. In particular, and relevant on a Gaelic course, Scots is strongly rhotic with a trilled r. This means that a Gaelic r does not resemble a Scots r, and you are highly like to hear something else. All I can say is that you have to get used to the way Gaelic is pronounced, which is really nothing like Scots at all. It might help to try to imagine the speaker is from London, and think 'how would a Londoner pronounce this r ?'
I have listened to all the examples listed for rudan and they all sound fine to me, both with my Gaelic ears on and with my English ears on, but I don't have Scots ears available.