Let's begin by looking at the most common article/sound pattern. Unfortunately, it is also the most complicated one. I will first list what happens with each sound - the "rules" so to speak. Next, I'll give examples, and finally I'll add some notes that will hopefully help further with sorting these out in your brain, as well as information on where this group actually gets used.

In the rules list, I will use L as an abbreviation for lenition. Lenition is when you add an h in writing after the first consonant of a word. In pronunciation, this means that the first consonant changes to a different consonant. Unfortunately for learners, lenition of l, n and r is not written in modern orthography. Very often, teachers will even tell you they don't lenite, but they do. Just not in writing.

You will notice that a number of rules are just "an before XY." I split them up, because it's easier to group the sounds that way and explain what happens. You are welcome to make your own list where you bunch these all together, once you have understood how it works.

The rules:

1. an before vowels
   
2. an before d t
   
3. an before l n r (but pronounced as a' + L or an + L)
   
4. an t- before sl sn sr + vowel (the s lenites to h or disappears when you pronounce it)
   
5. an before sg sm sp st
   
6. an + L before f
   
7. a' + L before all other consonants.

Examples:

1. Seo an obair
   
2. Seo an deoch
   
3. Seo an leabaidh
   
4. Seo an t-sràid
   
5. Seo an smaoin
   
6. Seo an fhìrinn
   
7. Seo a' chailleach

Notes:

1. Vowels always form a group - hooray!
   
2. Note that the n in an is pronounced in the same place as the d in deoch: tip of the tongue to the back of your teeth. So what, you ask?
   
3. l n r are a class of their own. The system is a bit messed up. In the beginning, memorise the writing and keep in mind that they do lenite. When learning how these sounds work overall, don't tackle them all at once. That's what I did and I got confused. For example, do only the n's until you've got them, then the l's etc.
   
4. Now, how on earth do I memorise when the t- goes before the s? Easy peasy, look at the previous group. Right, that's l n r, so all you did was add an s before them! And then you've got s + vowel. Done. And another note on pronunciation: Since t-sn turns into tn because the s disappears, another pronunciation rule kicks in and it is pronounced as nasal tr in most dialects.
   
5. Why don't these add a t-? Well, s lenites to a very soft h after an t-, virtually disappearing. Try pronouncing tg tm tp tt and I think you get the general idea why this is not done ...
   
6. f just generally likes to do its own thing. You will hear my evil laughter when you get to verbs and past tense.
   
7. And, last but not least, the basic pattern for this group! This is why I like to call it the "lenition" group. Keep in mind you're not "adding an h" but changing the sound of the consonant before the h. Kind of like in English with sit or shit. See, the sound of the s changed! And stop trying to put soap in my mouth. Oh, see the th in mouth and what the h does to the t there?

All the card-carrying members of the "lenition" group are nouns in the singular. All the examples above are feminine nouns in the basic case (or common case, or nominative/accusative case - yes, that's right, linguists don't agree on what to call the bloody thing). It is the most common article pattern though and is also used in the genitive case of masculine nouns, and prepositional (or dative) case for both feminine and masculine nouns. My advice is to study this group sound by sound until you understand how it works before you move on to the next group. And smile, because it gets a lot easier from now on!

Luckily, the rules for this group are a lot less complicated than for the first one. So, time to relax. Trust me, the worst is over.

The rules:

1. na h- before vowels
   
2. na before consonants
   

Examples:

1. Seo na h-obraichean
   
2. Seo na sràidean
   

Notes:

Seriously, that's it! So, no more to tell except on where this gets used: singular feminine nouns in genitive case and plural nouns in basic (common, nominative/accusative) or prepositional (dative) case, both masculine and feminine. The examples are in plural basic case.

This group usually gets presented with two rules only, but it is three rules really.

The rules:

1. nam before b p f m
   
2. nan before g c but pronounced as nang
   
3. nan everywhere else

Examples:

1. Seo taigh nam bodach
   
2. Seo taigh nan cailleachan
   
3. Seo taigh nan duine

Notes:

Why did I say that you usually get presented with only two rules? Well, that's because our learnéd society is in love with the written word and just lumps rule 2 into rule 3, neglecting to tell you about the difference in pronunciation! So, students try their best to memorise the seemingly arbitrary b p f m. There is nothing arbitrary about it though: those are the consonants you make with your lips, and nam merely moved its consonant to the same place, that's all. The same goes for c g and "nang" - the consonant in the article moves to where the first sound of the following word is. Gaelic article forms are all about the sounds! But as it happens, the Latin alphabet doesn't have a single letter for "ng," so it doesn't show up in writing. And finally, the usage: plural genitive nouns, both feminine and masculine.

This is the last group. It is very similar to the "nan nam" group.

The rules:

1. am before b p f m
   
2. an before g c but pronounced as ang
   
3. an before all other consonants
   
4. an t- before vowels

Examples:

1. Seo am bodach
   
2. Seo an cù
   
3. Seo an duine
   
4. Seo an t-eilean

Notes:

Just like in the previous group, the n in an goes where the following consonant is. Plus an added rule for vowels, just to keep you on your toes as a parting gift!

In some dialects, the article in this group affects the pronunciation of p b t d c g (so-called plosives or stops). If you are aware of this, it helps you understand people, even if you're not aiming for those dialects yourself. Read about it on the Akerbeltz site.

Usage: singular masculine nouns in the basic case only.

Here's a few final things to note:

• In the plural, there is no difference between masculine and feminine nouns.
  
• If you took the classical approach to learn by gender, case and number, you would have 3 singular cases x 2 genders = 6 patterns, plus 3 plural forms = 9 patterns! By looking at the sound patterns first and then at where they are used, we have reduced this to a mere 4 patterns! 9 for the price of 4, not bad, eh? Now you know why I insisted on the sounds so much in the beginning. By the way: you can use a similar strategy for learning the cases with nouns and adjectives, although they group differently.
  
• Make yourself lists of examples. They will help you learn the pattern, and then you will have a set of handy expressions all ready to use.
  
• Where is the vocative case, you ask? Well, the vocative a isn't really an article (which is why you don't spell it as a' with an apostrophe, actually), so that's a topic for when you study the cases.
  
• The feminine singular genitive article na is very distinctive with singular forms. Stick it in a useful phrase and use it to help you memorise the gender of nouns.

Last but not least, feedback and questions are welcome!