What is noticeable is that there has been very little book recommendation on this course, and indeed few people asking, in stark contrast to the Welsh course. I think there are two things that stop me recommending a book, and make me recommend the notes and discussion instead.
One is that the Gaelic taught here is very colloquial and actually has differences from any book I know of. There are sometimes discussions about what is used and thus what should be taught, but the fact remains that there is a discrepancy between this course and the books. This course does not seem to be based on any specific standard. This contrasts with the Welsh course, where there are two official courses in the colloquial language (one north and one south) that Duolingo accepts as definitive (good in a way, but you sometimes feel they are trying to teach two languages as once). There is also a good textbook that covers colloquial Welsh quite well in a reference-book style.
The other problem is that languages are complicated, with lots of tiny details that are easy to leave out. Languages such as English and French have had enormous resources spent on them, and there are ginormous tomes available that are essentially complete. Then these get distilled to make the books for learners. But in languages like Gaelic (Welsh not as bad), there is no definitive tome. There are bits where it seems no one has actually answered the question of 'what happens if...?' I have had discussions both here and on the Welsh site, where we have had to figure out the rule for the bit that is simply not complete in the books. So in some cases, the most complete answer is actually to be found in these discussion pages.
So I think the best thing is to read the notes and the discussion pages, and ask when you get stuck. Not only is there a group of people here who can advise or even work out if they don't know, but Duolingo seems to have another unique advantage. There are things that have been taught a certain way for years, even if it is confusing. There is one particular thing in Gaelic that always causes confusion, which I think is due to the teaching method being over 1000 years old and being designed for Latin speakers not English speakers. Duolingo contains a permanent record of the feedback from learners. It is obvious which bits are causing confusion, and people try different ways of explaining it until they find one that works.
It may be that someone else can recommend a recent book - I have not bought one for 10 years. But overall we are very lucky with the resources we have here. In particular the level of expertise you find on the discussion pages from the mods and others is better than any other language I know of.
It's just that certain words have this habit of inserting an h- before vowels. The fancy term is 'h-epenthesis'. You just have to learn them. It may help a bit with the memorizing to note that if you ignore one irregular form that most people don't even notice (so I won't give it) you can safely say that words that insert the h- are never the same words that cause lenition.
You will in fact meet different uses of the word na later, and they each have their own rules, but that is for another day.
As for why, no one really knows, except that it is a remnant of a much more complicated set of mutations that used to exist and that you can still find by doing the Duolingo Welsh course. If you go to their notes here and search in the text for
h- (with a space at the beginning) you will get about twenty hits, mostly describing all the situations where you add the h.
When you are pluralising a word beginning with a vowel, you need to add 'na h-' (the plural article). See the tips section under 'Travel' (you have to go to the website to see this, for some reason you can't see them on the app). Also look up the tips for 'The vocative case' under 'Names', specifically the section on vowels.
Most/all of your grammar questions up until this point of the course should be covered by the 'Tips' sections under each skill. When I realized this, I went back to the beginning to fill in my knowledge gaps.
Hey, Duolingo developers - how about making the tips in this course available on the app as well as the website please, like they are in some of the other languages?
This causes a lot of confusion because the sentences are so similar in Gaelic, but so different in English. But fundamentally you left out the definite article, which is what changes the sentence completely:
Tha na h-èisg anns an loch.
The fish are in the lake.
Tha èisg anns an loch.
Fish are in the lake. (literal)
There are fish in the lake. (good English)