Good question! Because Japanese originally has the word "kyō" for "today", and 今日 in (ancient) Chinese also means today. Japanese adapted Chinese characters (so called Kanji in Japanese) later, making this word pronounced "kyō" but written as "今日". There are some words in which the pronunciation is not related to the individual Kanji, but don't worry, such words are limited in Japanese. If you want to learn more, these words are called "熟字訓／じゅくじくん/Jukujikun" in Japanese.
Actually, 今日 was originally pronounced kepu in Old Japanese and WRITTEN as けふ, and *kyapu in Proto-Japonic (but that is sort of irrelevant here). Back to けふ. In the past, ふ represented the sounds pu, bu, and fu, just like how は represented the sounds wa, ha, pa, ba.
As time went on, and people became lazy, they stopped pronouncing the "p" sound of the ふ (ぷ). It ended up sounding something akin to けう, but closer to きょう. After the general orthographic reform following World War II, they decided to make the spelling reflect the pronunciation more, and declared the official spelling of 今日（けふ） as 今日（きょう).
A lot of きょう pronunciations today are corruptions of Go-on きゃう or けう, or are directly Han-on きょう.
今日: けぷ -> きょう
教: けう -> きょう
京: きゃう -> きょう
興: きゃう -> きょう
境: きゃう -> きょう
経: きゃう -> きょう
And the opposite case, where certain kanji are used for their sounds (readings) but the meanings of those kanji don't relate to the meaning of the word at all, are called 当て字 (あてじ), "ateji". One notable example of ateji is the kanji for sushi: 寿司.
In modern Japanese, ateji (当て字, 宛字 or あてじ, pronounced [ate(d)ʑi]; "called upon characters") principally refer to kanji used to phonetically represent native or borrowed words with less regard to the underlying meaning of the characters. This is similar to man'yōgana in Old Japanese. Conversely ateji also refers to kanji used semantically without regard to the readings.
For example, the word sushi is often written with its ateji 寿司. Though the two characters have the readings su and shi respectively, the character 寿 means "one's natural life span" and 司 means "to administer", neither of which has anything to do with the food. Ateji as a means of representing loanwords has been largely superseded in modern Japanese by the use of katakana (see also Transcription into Japanese), although many ateji coined in earlier eras still linger on.
Due to the number of homophones in Japanese, short words like this can mean several different things. Without either kanji or context to help define, I don't know if it wants sutra, interest, misfortune, or today. Even if the kanji is not the focus of the lesson, just having it there can do a LOT to help ground and reinforce learning words in Japanese.
The う after the sounds u and o makes it longer. The same happens with い and the i sound and the e sound. And with あ and a. So in this case きょ and きゅ are short and きょう、きゅう are long. Also, the small よ、ゆ、や after an i sound eliminate it doing the last consonant be followed by a y.
By "shorter" and "longer" do you mean the vowel sound changes some (like how english long and short vowels sound different) or do you mean the length of time you say the vowel changes? I've seen a few people say the う makes the vowel longer but I'm not toally sure which they mean.
It means time duration. Technically long vowels in english are not long vowels but diphthongs, historically they were, but as you can see from your orthography, the language has evolved but not actualized its terminology. In japanese, the vowels are still long in its original definition: Time. Actually, when learning a new language, always think of length as time, not as quality except you're told otherwise.
(Its a little long)
Okay after scrolling through comments, kanji was adapted from the Chinese and simplified by the Japanese. If you were to form a scentence using kanji, you use the ons reading. Which is basically the Chinese reading. Its why 今日 is pronounce kyo, you dont take the literal meaning from each kanji. Because by its self 日 means day and sun. (I am a beginner Japanese learner) I was confused about the kanji-so i looked it up and did research into radicals and the History of the Japanese language, alphabets, and how it works.
Please correct me if im wrong but this is what multiple websites have stated and actual Japanese people have said i may be wrong- but to help yall along Japanese is really easy, its mostly a context language. Ever listen to a Japanese show and wonder how does that one word, mean all of these english words? 食べる means eat. If placed into context with the right particals- in proper english it means "do you want to eat?" while a literal meaning is "eat." If you want to understand Japanese better as a whole- look up things you need to know.
Yes that's right. Small kana are often used to change the vowel sound. For example, chi + yu = chu (ち + ゆ = ちゅ).
When you see a small つ, that is called a glottal stop. This means that you take the following consonant sound as double. For example, がっこう would be pronounced gakkou, not gakou or gatsukou.
The major difference between hiragana and katakana is the fact that hiragana is primarily used to represent Japanese words, while katakana represents foreign words.
Japanese is a language with many borrowed words, and katakana immediately alerts the reader to the fact that the word is an imported one.
Hiragana examples: あか, りんご, ふゆ.
Katakana examples: アメリカ, ジョン, バター.