Vowels were implied in texts, because the texts were effectively known by heart, and the pronunciations were given orally, so there was never any real need to write them down. Eventually, after many centuries, a method of adding vowels into the text was developed called 'vowel pointings' or niqqud. This is employed in modern Hebrew, but the classical Hebrew that we use in the Golden Dawn does not use these.
Aleph and Ayin
The majority of people who encounter Hebrew for the first time in the Golden Dawn think the letter Aleph represents the English vowel 'a'. It's easy to see why people would think this and it's not helped by the fact that the First Knowledge Lecture puts down 'a' as the power of Aleph.
The truth is, however, that Aleph is a silent consonant, or, depending on the context, a glottal stop, which to most English speakers is pretty much the same thing, since we find it difficult to pronounce glottal stops. So why does it exist? It acts as a place-holder for when a vowel is implied but isn't obvious from the text.
For example, in the word Adonai () we see an initial Aleph, which doesn't actually represent the first letter 'A', but hints that a vowel precedes the Daleth. This stops us otherwise pronouncing the word as Donai.
You might notice that the second 'a' in the word doesn't require an Aleph. That's because it's not needed, since the vowels are implied between the other consonants of Nun and Yod. Esoteric students have an awful habit of putting in Alephs everywhere they see the letter 'a' in a transliterated word, but generally speaking you never put an Aleph in unless it's unclear that a vowel goes there. Most often it will be at the beginning of a word, but it also crops up elsewhere. There are exceptions to the rule, where an Aleph isn't really needed but shows up anyway, but these are occurrences are rare.
Ayin is similar to Aleph in that it's also a silent consonant or a glottal stop, and it acts as a place-holder for a vowel sound. You'll most likely see it in the middle of a word where you encounter a sharp stop in the vowel, followed by the same vowel, such as in the word Da'ath (). However, you can also find it in other places, such as the beginning of a word, like Aleph.
An important point I'd like to make now is that the glottal stop that we see in Da'ath is very different from the sound we encounter in the likes of ha-Aretz (). If these were one word, instead of two, it would most likely employ an Ayin for the double 'a', but since it's two words we need to approach them separately, where we recognise a vowel place-holder (in this case, Aleph) doesn't need to follow the initial Heh, but must precede the Resh in the following word.
Yod and Vav
Yod and Vav are also consonants, but they are more unusual in that they can have a hard consonantal sound ('y' and 'v') or can be place-holders for usually very specific vowel sounds ('i' and 'o' or 'u').
Generally speaking if either begin a word they will be pronounced as a voiced consonant, such as in the words Yesod () or Vav (). When they are found within in the middle of a word they are generally silent, indicating a vowel sound, such as in the words Michael () and Hod ().
The handy thing about these letters is that they tend to allows mark the place of specific vowel sounds. For example, Yod is generally pronounced 'ee', while Vav is generally pronounced 'oh' (long 'o') or sometimes 'oo'. This is why getting the pronunciation of words right is vitally important, as it will tell you if there is a Yod or Vav there. For example, the on part of Metatron () is not pronounced like the English word 'on', but rather like the word 'own' instead. This tells us that there is a Vav between the Resh and Nun. The same applies for the Yod in Michael. We pronounce it Mee-cha-el, not Mick-ah-el, because of this Yod.
There are exceptions to the rule, however, such as in the word Elohim (). The 'oh' sound is present, yet the Vav is not. If I did not already know what the word was and how it should be pronounced I'd assume it was supposed to be Elhim or Elehim (a shorter vowel sound), but this is something that we just need to learn. This occurence is not particularly common, but irregularities are found in all languages (English is full of them). In Hebrew these words that don't follow the rules usually carry extra significance worth meditating on.
When a Yod is found at the end of a word it is often pronounced as a diphtong (a double vowel), most commonly 'ai' (like in the word Thai). For example, Adonai is not spelled Adoni or pronounced Ah-doh-nee, but rather Ah-doh-nie. Other examples are Chai (), which means 'life', and Shaddai (), which means 'almighty'. Note how there's no Aleph before the Yod.
There aren't many words besides Vav that start with the letter Vav, so it almost always acts a place-holder for a vowel. The reason for this is because the letter is also used as a preposition, meaning 'and'. For example, 'light and life' is aur va-chaim (). Whenever you see a Vav at the beginning of a word it's usually safe to assume it means 'and', but context is always key. Whenever you see a word transliterated with a 'v', it's usually the letter Beth instead (for example, Levannah - ), which can be pronounced as both 'b' and 'v' (more on that in a future post).
One thing that usually throws people is when they encounter an Aleph or Ayin with a Vav at the beginning of a word, such as in the word Olam (). Instinctively we would spell Olam as Vav Lamed Mem, since we're using the Vav to hold the place of the long 'o' sound, but when we look at it logically we realise we cannot do this, as the Vav would be misconstrued as the hard 'v' sound, ending with a word like Valem (which, as far as I'm aware, doesn't actually exist). So we have to add a place-holder, to indicate that the Vav is silent and therefore pronounced as a vowel.
Feel free to ask questions in the comments section. Part 2 will cover letters with two sounds, doubling up of letters, and a few basic prepositions, including why they always join the following word. If there are any particular areas of Hebrew you'd like covered, feel free to ask.