I don't get it. Why is these open but this closed? It sounds like some weird grammatical blue law.
It has nothing to do with grammar. It's pronunciation. The syllable types aren't 'rules' per se - they're just a breakdown of different patterns. The complications come more from how we write down the sounds.
Once again, 'these' Is NOT an open syllable. It's a third, different type. More on that, below.
From a linguistics perspective, English phonemes (the smallest unit of sound that has a meaningful difference) are either vowels or consonants. English has approximately 44 phonemes (regional variants might have a couple more or less), yet only 26 letters in the alphabet to represent them.
Those approximately 44 phonemes for English break down to about 24 consonant sounds, leaving roughly 20 vowel sounds. The number of vowel sounds varies far more by dialect and variant than the consonants do. The general AmE accent for example has only around 14-16 vowel sounds, while Received Pronunciation (i.e. "standard") BrE) has 20-25. AusE has around 19-20 vowel sounds. Regardless of dialect, there are still only six letters (if you count y) in the alphabet to represent those 20-ish vowel sounds, leaving you with only 20 more to represent those 24-ish consonant sounds.
A syllable is single, unbroken sound part of a word, comprised of any number of phonemes. The number of syllables in a word is unrelated to the number of total phonemes that comprise it. For example, 'chug' is one syllable, but has three phonemes (ch/u/g), represented by four letters. We don't have a special letter for that 'ch' sound, so we represent it with two letters. 'Fight' is also one syllable, with three phonemes (f/igh/t), but we need five letters to write it. Note that vowel sound in it is represented with 'igh', but the same sound /aɪ/ can be represented with just the letter 'I' in 'price'. 'Price' still has only one syllable but four phonemes (p/r/i/ce) and yet five letters. English is not written very phonetically. It's simply a spelling convention that we spell it 'fight' instead of 'fite'. That's an entirely different conversation though.
In English, every syllable must have one vowel sound. Each time you hit a new vowel sound, you get a new syllable. There may or may not be consonants that accompany the vowel; when they do, they may come before, after, or on both sides of the vowel. Research has shown that despite all the hundreds if not thousands of possible vowel and consonant combinations in English, they can all be boiled down into only six syllable patterns, which are:
1. Open - no consonant on end, vowel is long.
Examples: me, the second syllable in hello, and the first syllable in 'item'
2. Closed -
ends(but may also begin with) a consonant, and has a short vowel sound.
Example: it, cat, hit, pot (and from above', 'this")
3. Vowel-Consonant-
E (aka silent e, sneaky e, bossy e, etc.) - just as the pattern says, it's a vowel followed by a constant ending in the letter 'e' which is silent.
Examples: fine,cake,Pete (and from above, 'these' and 'those')
4. Vowel team - two vowels work together to produce one vowel sound.
Examples: steam, boil
5. R-controlled
- the letter r follows a vowel, and colors/controls/influences the vowel sound to where it's neither long nor short.
Examples: star, cord,skirt
6. Consonant-L-E - as the pattern says, the word ends with a consonant followed by the letter L and a final E, which results in a schwa sound rather than a long/short 'e'
Examples:table, bubble, circle
Some make a distinction with the fourth pattern, and separate the diphthongs (two vowels blending into a new sound such as
boil, loud) from the digraphs (two vowels making one sound such as
meat, float) into a seventh syllable type. Again, that ties back to spelling conventions and the lack of unique letters in our alphabet to represent the total number of sounds in our language.